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The effect of polar substituents on the heterocyclic benzoxazoles
Tetrahedron 62 (2006) 9383–9392
The effect of polar substituents on the heterocyclic benzoxazoles Chung-Shu Wang,a I-Wen Wang,a Kung-Lung Chengb and Chung K. Laia,* a
Department of Chemistry and Center for Nano Science Technology, National Central University, Chung-Li 320, Taiwan, ROC b Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC Received 22 May 2006; revised 3 July 2006; accepted 20 July 2006 Available online 10 August 2006
Abstract—The synthesis, characterization, and mesomorphic properties of a new type of heterocyclic compounds 1, 2 derived from benzoxazole are reported. In order to understand the relationship between the structure and the mesomorphic behavior, compounds containing a variety of polar substituents (i.e., X¼H, F, Cl, Br, CH3, CF3, OCH3, NO2, CN, OH, NMe2, COOCH3) on the terminal end were prepared. The phase behavior of these mesogenic compounds was characterized and studied by differential scanning calorimetry (DSC) and polarization optical microscopy. The formation of mesophases was strongly dependent on the electronic and/or the steric factors of the substituents. In general, a mesophase was better induced by introduction of a polar substituent. Compounds (X¼H) formed a crystalline phase, however, other compounds, except for X¼OH, exhibited nematic or smectic A phases. Interestingly all compounds with electron-donating substituents (X¼CH3, OCH3, NMe2) exhibited nematic phases, however, other compounds with electron-withdrawing substituents (X¼F, Cl, Br, CF3, NO2, CN, COOCH3) formed smectic A phases. Compounds (X¼NO2, CN, COOCH3) have higher clearing temperatures than those of other homologues, and the higher Tcl was attributed to an enhanced conjugative interaction. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett sp constants was found. The fluorescent properties of the compounds were examined. All lmax peaks of the absorption and photoluminescence spectra of compounds occurred at ca. 348–381 and 389–478 nm, respectively. Whereas, the quantum yields of some compounds were relatively low. Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction A delicate balance of molecular interactions in a specific state1 is crucially essential to the formation of a predesigned structure. This is particularly important in liquid crystalline materials, and liquid crystals form a state of matter intermediate between the solid and the liquid. This type of molecular interaction1c,d induced to form mesophases can be weak dipole–dipole interaction, dispersion, H-bonding, coordinative force, and others. Forces which are too strong or too weak can lead to the formation of a solid or liquid state. Numerous heterocyclic compounds2 derived from compounds such as 1,3,4-oxadiazole,3 1,2,4-triazole,4 and benzoxazole5 exhibiting interesting mesophases have recently been explored due to their varieties in structures and/or known chemistry. Better mesomorphic behavior6 formed by these heterocyclic structures was attributed to their unsaturation and/or more polarizable nature. On the other hand, a lower symmetry3 and/or non-planar structure caused by nitrogen, oxygen, sulfur, or other atoms incorporated on
* Corresponding author. Tel.: +886 3 4259207; fax: +886 3 4277972; e-mail: [email protected] 0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.07.058
such heterocyclic rings also explained the preferred or better mesomorphic properties. Materials with lower melting temperatures, potential candidates for further applications, are often generated by such compounds. It is well known that the stability7 of the mesophase may be augmented by an increase of the polarity or polarization along the mesogenic core of the molecule. A substantial change of the micro- as well as macro-polarizability in a specific structure or molecule can be easily achieved by the introduction of polar substituent along the preferred molecular direction. The effect of polar substituents in a variety of mesogenic systems8 has been studied and investigated during the past years. Some predictions were successfully made in terms of mesomorphic behavior. The transition temperature of M/I has been correlated with the polarizability anisotropy of bonds to the substituent in 4-(4-substituted phenylazo)phenyl-4-alkoxybenzoates.8d Previous studies7,8c also showed that compounds substituted with polar groups, such as –NO2, –CN, might lead to a higher clearing temperature, which has been attributed to the conjugative interaction increase between the substituent and the ester moiety. On the other hand, the effect of polar substituents on the mesogenic 1,3,4oxadiazoles-based materials was also studied. The mesophase and optical properties of these oxadiazole materials3a were also found to be strongly influenced by the presence of a terminal polar group.
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C.-S. Wang et al. / Tetrahedron 62 (2006) 9383–9392 OR
C12H25O
X
RO
b
C12H25O
c
Y
C12H25O
NO2
NH2
OH
OH
N N
OH
RO O
O
O Cu
O
O
OR
d
a
X
H2n+1CnO
HO
OH
X = Me, OMe, F, Cl, CN, NO2
NO2 C12H25O
Y
OH
OR X
N e
OR
R = C12H25; X = H, F; Y = H, F, CF3, OCH3, NMe2
Benzoxazoles, as an important type of heterocyclic compound, have been studied in a variety of research areas, including non-linear optics (NLO),9 organic light-emitting diodes (OLED),10 and polymeric materials.11 However, benzoxazole derivatives exhibiting mesophases5a,12 have been less often reported. Nematic or/and smectic C mesophases formed by these rod-like compounds were observed. Benzothiazole-derived compounds were also found to have photophysical and fluorescent applications.13 In a previous paper, the substituent effect on the columnar formation in b-diketonate metallomesogens14 was investigated. A rectangular columnar phase (Colr) was observed for substituents with bulkier groups (i.e., X¼Me, Et), however, a hexagonal columnar phase (Colh) was observed for substituents with electron-withdrawing groups (i.e., X¼Cl, Br, I). Interestingly, a satisfactory linear relationship with a correlation coefficient of 0.974 between the clearing temperatures and Hammett sp constants was obtained for the copper complexes.
C12H25O
O NH2
f
C12H25O
O NO2
N
N
4
3
g C12H25O
O N N
X 1a-1l
X = H, F, Cl, Br, CH3, CF3, OCH3, NO2, CN, OH, NMe2, COOCH3
Scheme 1. Reactions and reagents: (a) RBr (1.1 equiv), KHCO3 (1.3 equiv), KI, refluxed in dry CH3COCH3, 48 h, 60 C; (b) HNO3 (1.2 equiv), NaNO2 (0.2 equiv), stirred at 0 C in CH2Cl2, 12 h, 25 C; (c) N2H4 (1.2 equiv), Pd/C (0.1 equiv), refluxed in absolute C2H5OH, 6 h, 85 C; (d) 4-Nitrobenzaldehyde (1.0 equiv), CH3COOH (drops), refluxed in absolute C2H5OH, 12 h, 90 C; (e) Pb(OAc)4 (1.3 equiv), refluxed in CH2Cl2, 6 h, 85 C; (f) N2H4 (1.2 equiv), Pd/C (0.1 equiv), refluxed in absolute ethanol, 6 h, 80%; (g) substituted benzaldehyde (1.0 equiv), CH3COOH (drops), refluxed in absolute C2H5OH, 12 h, 68–90 C.
2.1. Mesomorphic properties As part of our studies of heterocyclic mesogens, in this paper, we describe the synthesis, characterization, and the mesomorphic properties of a series of calamitic benzoxazole derivatives with various substituents on the terminal phenyl ring. Their fluorescent properties were also examined. Nematic or smectic phases were observed depending on the electronic nature of the substituents. However, the observed optical properties were not sensitive to the substituents except for the compound with an NMe2 group. 2. Results and discussion The synthetic procedures5a for the benzoxazole derivatives 1a–1l are summarized in Scheme 1. The majority of benzoxazole-based derivatives were prepared by the condensation of 2-aminophenols with benzaldehydes or benzoic acid and subsequent intramolecular cyclization. The compounds of 5-dodecyloxy-2-nitrophenol, 2-amino-5-dodecyloxyphenol, 5-dodecyloxy-2-[(4-nitrobenzylidene)amino]phenol, 6-dodecyloxy-2-(4-nitrophenyl)benzoxazole 3, and 4-(6dodecyloxybenzoxazol-2-yl)phenylamine 4 were prepared by reported procedures.5a The final compounds, benzylidene-4-(6-dodecyloxybenzoxazol-2-yl)phenylamine were prepared by the condensation reaction of 4-(6-dodecyloxybenzoxazol-2-yl)phenylamine and appropriate aldehydes in refluxing absolute ethanol with a yield ranging from 67–82%. 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis were used to characterize the intermediates and the final products.
The mesomorphic behavior of compounds 1a–1l was characterized and studied by differential scanning calorimetry (DSC) and polarizing optical microscopy. The phase transitions and thermodynamic data were summarized in Table 1. In order to study the substituent effect on the formation of the mesophases, a variety of compounds with a variety of polar groups (i.e., X¼H, F, Cl, Br, CH3, CF3, OCH3, NO2, CN, OH, NMe2, COOCH3) substituted on the opposite end of the benzoxazole ring were studied. Among these polar groups, some are known as strong p-acceptors (i.e., X¼CN, NO2) or p-donors (X¼Cl, Br, OH), and others are known as intermediate (X¼Cl, COOCH3), besides being classified as electron-donating or electron-withdrawing groups. The results indicated that the formation of mesophases in this type of electron-deficient heterocyclic system was strongly dependent on the electronic and/or the steric factors of the substituents. Compound 1a (X¼H) was in fact non-mesogenic. A transition of crystal-to-isotropic at 99.8 C was observed, and the lack of liquid crystallinity of compound 1a can probably be attributed to the insufficient dipole over the entire molecule. When a polar functional group was substituted on the phenyl ring located on the opposite side to the benzoxale core, the mesomorphic properties were improved. All compounds except for 1j (X¼OH) were truly mesogenic, in which introducing a polar group was used to enhance or/and induce the formation of the mesophases. The results also indicated that compounds (1e, 1g, 1k) with an electron-donating substituent (–CH3, OCH3, NMe2) exhibited nematic (N) phases, however, other
C.-S. Wang et al. / Tetrahedron 62 (2006) 9383–9392
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Table 1. Phase transitions and enthalpies of compounds 1a–1l
99.8 (60.1) Cr
1a
79.8 (60.8) 101.7 (59.2)
1b
Cr 82.4 (2.03)
1c
Cr1
1d
Cr1
Cr2
74.9 (56.7) SmA
Cr2
1e
Cr
1f
Cr
80.2 (13.1) 103.9 (55.4)
Cr1
1h
Cr1
1i
Cr1
92.0 (4.61)
Cr2
108.3 (51.6)
137.0 (0.40) 180.9 (6.22) SmA
88.1 (1.96) 102.9 (8.93) 86.3 (6.08)
Cr2
I
I
179.5 (6.11) 169.5 (0.49)
N 169.2 (0.47)
129.3 (7.85) Cr2
I
137.4 (0.38)
76.3 (51.5) 95.6 (2.28)
192.8 (6.36)
N
90.8 (47.6)
1g
I
194.3 (6.42) SmA
86.7 (56.5)
75.3 (50.4)
I
174.0 (4.24) 173.2 (3.28)
97.9 (59.9)
63.3 (14.6)
155.6 (4.25) 154.7 (4.12)
92.1 (45.3) 74.3 (10.3)
50.0 (25.5)
SmA
I
SmA 127.0 (8.03)
224.8 (2.64) 224.2 (2.66)
120.7 (40.1) SmA 102.7 (30.7)
I
219.1 (3.27)
I
I
217.3 (3.21) 130.7 (16.7) Cr
1j
1k
Cr1
123.3 (34.6)
SmX
156.7 (0.29)
Cr1
103.1 (35.6)
Cr2
115.2 (12.7)
SmA
I
161.0 (0.60) 160.5 (0.53)
156.2 (0.23)
111.8 (33.8)
133.0 (51.6) 1l
N
117.2 (16.4)
I
212.5 (5.68) 210.8 (5.54)
I
Cr1, Cr2¼crystalline phases, SmX¼unidentified smectic phase, SmC¼smectic C phase, SmA¼Smectic A phase, N¼nematic phase, I¼isotropic phase.
compounds (1b, 1c, 1d, 1f, 1h, 1i, 1l) with an electronwithdrawing substituent (–F, –Cl, –Br, –CF3, –NO2, –CN, –COOCH3) formed smectic A (SmA) phases. The N and SmA phases were observed and identified by optical texture, as shown in Figure 1. Among them, compounds 1b, 1c , 1d formed SmA phases, however, compound 1d containing a larger and/or more polarized group (X¼Br) enhanced the SmA phase more than compounds with a smaller and/or less polarized group (X¼F, Cl). The clearing temperature of compound 1d was higher than those of compounds 1b and 1c, i.e., 194.3 C>174.0 C>155.6 C. This order in clearing temperatures and the temperature of mesophase was parallel to the increased polarizability in the order of Br>Cl>F. On the other hand, the temperature range of the mesophase was also wider for compound 1d than those of compounds 1a and 1b, i.e., 96.4 C>81.9 C>53.9 C on heating. Interestingly, among these electron-withdrawing substituents, compounds 1h, 1i, 1l (X¼NO2, CN, COOCH3) appeared to have clearing temperatures higher than those of other homologues, which ranged from 212.5 and 224.8 C. These polar groups are also known as good p-acceptors, and have a more planar structure (i.e., sp or sp2 hybrid orbital). The increase in Tcl might be attributed to two factors. Electronically, a conjugative interaction over the entire molecule was better achieved between the polar substituent and the other side of benzoxazole, and also a better packing arrangement
due to a more overall molecular planar structure may be achieved in the solid and liquid crystal states. The temperature range of mesophases were relatively wide with these three compounds, at ca. 79.5 C (1l)–98.4 C (1i). Among these three substituents, the NO2 group is particularly worthy to note. Compound 1h (X¼NO2) has the highest clearing temperature in the whole series, and this is probably due to NO2 group having the largest dipole. The range of mesophase temperature is relatively high; 98.4 C (1i; CN)>96.4 C (1d; Br)>95.5 C (1h; NO2). The effect of lateral and terminal –NO2 as a substituent in achiral calamitic liquid crystals has been reviewed.8e The results indicated that substantial changes in clearing and/or melting temperatures were obviously observed depending on the molecular structures and the substituted position. A polar NO2 group attached to the hydrocarbon moiety changes substantially the electron affinity of the molecules, and the strong mesomeric effect caused by the NO2 group induces a positive charge on the nitrogen atom and increases the electronegativity of the group.8e,15 The transition enthalpies of SmA/I in the whole series of the compounds, measured with 2.64 kJ/mol (1h)–6.42 kJ/mol (1d), varied slightly with the substituents. On the other hand, compounds 1e and 1f, which are relatively equal in molecular size, exhibited a different mesophase; N phase for 1e and SmA phase for 1f, respectively. On the other hand, a higher Tcl in compound 1f (180.9 C) over 1e (137.4 C) by ca. 43.4 C, as well as a wider mesophase range by DT¼56.6 C
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N 150
SmA SmX
100
Cr 50 0
1a 1b 1c 1d 1e
1f
1g 1h
1i
1j 1k
1l
compds
Figure 2. Bar graph showing the phase behavior of compounds 1a–1l.
a transition of crystal-to-isotropic at 130.7 C was observed. The lack of mesomorphic properties might result from the intermolecular H-bonds formed, which can lead to a stronger molecular interaction between the molecules. Figure 2 showed the bar graph of the transition temperatures of compounds 1a–1l. The relationship between the melting temperatures, the clearing temperatures or the temperature ranges of the mesophases with the electronic properties of the substituents was plotted. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett sp constants for the substituents was found in this system (Fig. 3).
(i.e., 90.1 C for 1f vs 33.5 C for 1e) might be attributed to a better or larger dipole on 1f (CF3) over 1e (CH3). The electronic factor of the substituent, but not the steric factor, might have played a more important role in forming or inducing the phase. Heterocyclic benzoxazole is known as an electron-deficient moiety, and polarization or electron distribution by an electron-donating group, such as –CH3, is better achieved by compound 1f than by compound 1e. Compound 1g formed a N phase. The transition enthalpies of N/I, measured with 0.38 kJ/ mol (1e)–0.60 kJ/mol (1k), were insensitive to the substituents. Compound 1j (X¼OH) was non-mesogenic, and only
240 220
COOCH3
200
temp.
Figure 1. Optical textures observed by 1e (X¼CH3) at 119.0 C (top plate), 1f (X¼CF3) at 165.0 C (middle plate), and 1l (X¼COOCH3) at 175.0 C (bottom plate).
In order to understand the effect of terminal carbon chain length on the formation of mesophase, the compounds 2a (X¼OCH3) and 2b (X¼CN) with a carbon chain length of n¼8 and 16 were also prepared and studied. The phase transitions and thermodynamic data are summarized in Table 2. The mesomorphic properties exhibited by three compounds 2a and 1g with different carbon chain length (n¼8, 12, 16) were compared. Compounds 2a (n¼8) and 1g (n¼12) exhibited nematic phases, however, the compound 2a with a longer carbon chain length (n¼16) formed a N phase and a SmA phase. This phase behavior is expected for rod-like molecules. The melting temperature increased slightly with the carbon chain length, i.e., Tmp: 106.1 C (n¼8)< 108.3 C (n¼12)<112.5 C (n¼16), however, the clearing temperatures decreased with the carbon chain length, i.e., Tcl: 186.0 C (n¼8)>169.5 C (n¼12)>158.1 C (n¼16). The temperature range of the mesophase increased with carbon chain length, DT¼82.9 C (n¼8)>61.2 C>45.6 C (n¼16) (Fig. 4). Elongation of the overall molecular length by increasing the carbon length on the other side of terminal chains led to the formation of more ordered SmA phases beside for N phase at higher temperature. This can probably
180 160
2
Br
140
R = 0.6882
CF3
Cl
OCH3 NMe2
NO2
CN
F CH3
120 100 -0.4
-0.2
0
0.2
0.4
0.6
0.8
1
σp Figure 3. The plot of clearing temperature ( C) with Hammett sp constants of substituents in 1a–1l.
C.-S. Wang et al. / Tetrahedron 62 (2006) 9383–9392
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Table 2. Phase transitions and enthalpies of compounds 1g, 1i, 2a, and 2b
106.1 (43.6) 2a; n = 8
1g;
2a;
Cr
12
Cr1
16
Cr
92.0 (4.61)
Cr2
88.3 (67.6)
Cr
106.2 (27.3)
12
2b;
16
Cr1
N
SmA
187.0
a
86.3 (6.08)
I
I
157.0 (0.63) 233.0 (0.45)
N
I
232.2 (0.41) 219.1 (3.27)
120.7 (40.1) Cr2
169.5 (0.49)
158.1 (0.63) N
195.0a
I 185.3 (0.46)
169.2 (0.47)
153.7 (0.42)
102.9 (8.93) 1i;
186.0 (0.47) N
154.8 (0.43) SmA
126.3 (31.9) 8
108.3 (51.6) 76.3 (51.5)
112.5 (63.4)
2b;
78.0 (42.2)
SmA 120.9 (12.2)
Cr
I
217.3 (3.21)
102.7 (30.7) SmA 100.7 (78.8)
213.7 (6.16)
I
211.9 (6.09)
Cr1, Cr2¼crystalline phases, SmA¼Smectic A phase, N¼nematic phase, I¼isotropic phase. a Determined by optical microscope.
230
temp.
180
N SmA
130
spectra of all compounds 1a–1l were very similar in shape because of their structural similarity. The highest absorption peaks of all compounds were found to be insensitive to the substituents. Interestingly, they all occurred at ca. 348– 365 nm except for two compounds 1h and 1k, for which
Cr2 Cr1
80
0.6 1a 1b 1c 1d 1f 1h 1i 1l
30 0.5 2a(n=8) 1g(n=12) 2a(n=16) 2b(n=8) 1i(n=12) 2b(n=16) 0.4
compds
Figure 4. Bar graph showing the phase behavior of compounds 2a, 2b.
absorbance
-20
be attributed to an enhanced dispersive interaction between the terminal alkoxy chains. Similar mesomorphic behavior was also observed in compounds 2b (n¼8, 16) and 1i (n¼12), and compounds 2b with longer carbon chain length (n¼12, 16) favored the formation of the more ordered SmA over N phase.
0.3
0.2
0.1
0.0 300
350
400
450
500
wavelength (nm) H2n+1CnO
O 0.6
N N
1a 1e 1g 1j 1k
0.5 1g; 1h; 1i; 1j; 1k; 1l;
X = OCH3 X = NO2 X = CN X = OH X = NMe2 X = COOCH3
n = 8, 16 2a; X = OCH3 2b; X = CN
0.4 absorbance
1a; 1b; 1c; 1d; 1e; 1f;
n = 12 X=H X=F X = Cl X = Br X = CH3 X = CF3
X
0.3 0.2 0.1
2.2. Optical properties The UV–vis absorption and PL spectra of the compounds 1a–1l in CH2Cl2 solution are presented in Figures 5 and 6. The data of lmax peaks of UV–vis absorption and PL spectra in CH2Cl2 are listed in Table 3. The absorption and emission
0.0 300
350
400
450
500
wavelength (nm)
Figure 5. Normalized absorption spectra of compounds with electron-withdrawing (top) and electron-donating substituents (bottom) in CH2Cl2.
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300 250 200 intensity
quantum yields of some compounds in CH2Cl2 estimated with anthracene as a standard (4f¼0.27 in hexane) were relatively low. For example, the yields were all ranged between 0.05 and 0.07%.
1a 1b 1c 1d 1f 1h 1i 1l
150
3. Conclusions
100 50 0 400
450
500
550
wavelength (nm)
1a 1e 1g 1j 1k
150 125
The effect of polar substituents on a series of heterocyclic benzoxazoles was systematically studied, and the formation of mesophases was observed depending on the electronic and/or the steric factors of the substituents. In general, the mesophase observed was improved by introducing a polar group. Most compounds (except 1j) exhibited nematic or smectic A phases. Compounds with electron-donating substituents favored nematic phases, however, compounds with electron-withdrawing substituents formed smectic A phases. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett sp constants was found.
intensity
100
4. Experimental section
75
4.1. General
50 25 0 400
450
500
550
wavelength (nm)
Figure 6. Normalized PL spectra of compounds with electron-withdrawing (top) and electron-donating substituents (bottom) in CH2Cl2.
the lmax were all shifted to longer wavelength, by ca. 31 and 33 nm, respectively. The electronic properties of the substituents might not play an important role in the UV absorption spectra. The photoluminescence spectra measured in CH2Cl2 of compounds 1a–1l are also shown in Figure 6. A similar trend was also observed in the photoluminescence spectra. The emission peaks occurred at 386–409 nm except for compounds 1k, which the lmax were slightly shifted to longer wavelength, by ca. 80 nm. The dependence of PL spectra is more sensitive to the electronic factor than that of UV-absorption. However, it is worth noting that the
Table 3. A summary of UV and PL peaks for compounds 1a–1l Compounds sp 1a 1b 1c 1d 1f 1h 1i 1l 1e 1j 1g 1k
0 0.15 0.24 0.26 0.53 0.81 0.71 0.44 0.14 0.22 0.25 0.32
UV (nm)
PL (nm)
273, 313 (sh), 348 382, 398 273, 313 (sh), 348 382, 398 277, 313 (sh), 348 387, 397 277, 313, 352 392 267, 313, 356 372, 389 283 (sh), 309, 379 391 (sh), 409, 425 (sh) 273, 309, 365 391 (sh), 409, 425 273, 309, 362 372, 389 280, 313 (sh), 348 382, 398 280 (sh), 313 (sh), 348 372, 386 284 (sh), 313 (sh), 348 382, 398 319 (sh), 381 478
All chemicals and solvents were reagent grades from Lancaster or Aldrich. Solvents were dried by standard techniques. 1H and 13C NMR spectra were measured on a Bruker AM-300. FTIR spectra were performed on Nicolet Magna-IR 550 spectrometer. DSC thermographs were carried out on a Mettler DSC 821 and calibrated with a pure indium sample (mp¼156.60 C, 28.45 J/K), and all phase transitions are determined by a scan rate of 10.0 /min. Optical polarized microscopy was carried out on an Zeiss AxiaPlan equipped with a hot stage system of Mettler FP90/ FP82HT. The UV–vis absorption and fluorescence spectra were obtained using HITACHI F-4500 or JASCO V-530 spectrometer, and all spectra were recorded at room temperature. Elemental analysis for carbon, hydrogen, and nitrogen were conducted on a Heraeus Vario EL-III elemental analyzer at National Taiwan University. 3-Alkoxyphenols, 5-alkoxy-2-nitrophenols, 2-amino-5alkoxyphenols, 5-alkoxy-2-[(4-nitro-benzylidene)amino]phenols, 6-alkoxy-2-(4-nitrophenyl)benzoxazoles, 4-(6-alkoxybenzoxazol-2-yl)phenylamines were prepared by similar procedures.5a 4.1.1. 3-Dodecyloxyphenol5a (n¼12). White solid, yield 60%. 1H NMR (CDCl3): d 0.87 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.43 (m, 18H, –CH2), 1.72–1.77 (m, 2H, –OCH2CH2), 3.90 (t, 2H, –OCH2, J¼6.6 Hz), 6.39–6.40 (m, 2H, Ar-H), 6.46–6.48 (m, 1H, Ar-H), 7.10 (t, 1H, Ar-H, J¼8.5 Hz). 13 C NMR (CDCl3): d 14.14, 22.71, 26.05, 29.23, 29.37, 29.42, 29.60, 29.63, 29.66, 29.68, 31.94, 68.10, 102.08, 107.10, 107.59, 130.10, 156.68, 160.51. 4.1.2. 5-Dodecyloxy-2-nitrophenol5a (n¼12). Yellow solid, yield 25%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.44 (m, 18H, –CH2), 1.75–1.80 (m, 2H, –CH2), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.47–6.49 (m, 2H, Ar-H), 8.00 (d, 1H, Ar-H, J¼10.1 Hz), 11.02 (s, 1H,
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Ar-OH). 13C NMR (CDCl3): d 14.12, 22.69, 25.87, 28.83, 29.27, 29.35, 29.41, 29.52, 29.57, 29.63, 29.64, 29.71, 31.92, 69.15, 101.80, 109.83, 126.91, 127.54, 158.01, 166.74.
(CDCl3): d 14.15, 22.71, 26.08, 29.28, 29.37, 29.43, 29.60, 29.62, 29.66, 29.68, 31.94, 68.88, 96.13, 112.74, 114.70, 117.27, 119.17, 128.93, 135.96, 149.26, 151.35, 157.19, 162.94.
4.1.3. 2-Amino-5-dodecyloxyphenol5a (n¼12). White solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.41 (m, 18H, –CH2), 1.68–1.73 (m, 2H, –CH2), 3.84 (t, 2H, –OCH2, J¼6.6 Hz), 6.32 (dd, 1H, ArH, J¼2.6 Hz, 8.6 Hz), 6.41 (d, 1H, Ar-H, J¼2.7 Hz), 6.75 (d, 1H, Ar-H, J¼8.6 Hz). 13C NMR (CDCl3): d 13.94, 22.59, 26.02, 29.25, 29.34, 29.51, 29.53, 29.56, 29.58, 31.85, 68.67, 102.72, 106.75, 120.85, 125.58, 148.06, 154.94.
4.1.7. Benzylidene-4-(6-dodecyloxybenzoxazol-2-yl)phenylamine (1a; n¼12, X¼H). A mixture of benzaldehyde (0.10 mL, 0.001 mol) and 4-(6-dodecyloxybenzoxazol-2yl)phenylamine (0.4 g, 0.001 mol) was dissolved in 100 mL of absolute ethanol under nitrogen atmosphere. The solution was refluxed for 10 min, and 0.5 mL of acetic acid was slowly added. The solution was refluxed for 12 h. The brown solids were then collected. The products isolated as light yellow solids were obtained after recrystallization from C2H5OH, yield 78%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.8 Hz), 1.25–1.47 (m, 18H, –CH2), 1.80– 1.83 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, Ar-H, J¼2.3 Hz, 8.7 Hz), 7.09 (d, 1H, Ar-H, J¼2.3 Hz), 7.31 (d, 2H, Ar-H, J¼8.4 Hz), 7.47–7.50 (m, 3H, Ar-H), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.91–7.92 (m, 2H, Ar-H), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.49 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.08, 29.26, 29.36, 29.42, 29.60, 29.62, 29.66, 29.68, 31.94, 68.90, 96.08, 113.33, 119.81, 121.44, 124.88, 128.29, 128.87, 129.04, 131.81, 135.86, 135.94, 151.67, 154.42, 157.78, 161.20, 162.04. IR (KBr): 2919, 2848, 1625, 1614, 1505, 1496, 1490, 1467, 1281, 1226, 1150, 1053, 1004, 968, 861, 840, 800 cm1. MS (FAB): calcd for C32H39N2O2: 482.3, found: 483.4 [M+H]+. Anal. Calcd for C32H38N2O2: C, 79.61; H, 7.79; N, 5.42. Found: C, 79.63; H, 7.94; N, 5.80.
4.1.4. 5-Dodecyloxy-2-[(4-nitrobenzylidene)amino]phenol5a (n¼12). Orange solid, yield 90%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.8 Hz), 1.25–1.44 (m, 18H, –CH2), 1.74–1.79 (m, 2H, –CH2), 3.94 (t, 2H, –OCH2, J¼6.5 Hz), 6.47 (dd, 1H, Ar-H, J¼2.4 Hz, 8.9 Hz), 6.55 (d, 1H, Ar-H, J¼2.5 Hz), 7.31 (d, 1H, Ar-H, J¼8.9 Hz), 7.40 (s, 1H, Ar-OH), 7.99 (d, 2H, Ar-H, J¼8.6 Hz), 8.29 (d, 2H, Ar-H, J¼8.6 Hz), 8.66 (s, 1H, Ar–CH]N–Ar). 13 C NMR (CDCl3): d 14.12, 22.70, 26.02, 29.16, 29.36, 29.38, 29.58, 29.60, 29.65, 29.67, 31.93, 68.39, 100.57, 107.84, 116.45, 124.11, 127.46, 128.73, 141.71, 148.90, 149.83, 154.66, 161.56. 4.1.5. 6-Dodecyloxy-2-(4-nitrophenyl)benzoxazole5a (3; n¼12). The mixture of 5-dodecyloxy-2-[(4-nitrobenzylidene)amino]phenol (3.2 g, 0.007 mol) dissolved in 125 mL of CH2Cl2 and Pb(OAc)4 (4.32 g, 0.01 mol) was refluxed for 4 h. The solution was extracted with 100 mL of CH2Cl2/H2O (1/2), and the organic layers were combined and dried over MgSO4. The solution was concentrated and the brown solids were recrystallized from C2H5OH. The light yellow solids were isolated with a yield of 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.49 (m, 18H, –CH2), 1.74–1.84 (m, 2H, –CH2), 4.01 (t, 2H, –OCH2, J¼6.5 Hz), 6.99 (dd, 1H, Ar-H, J¼2.28, 8.8 Hz), 7.09 (d, 1H, Ar-H, J¼2.2 Hz), 7.65 (d, 1H, Ar-H, J¼8.8 Hz), 8.34 (m, 4H, Ar-H). 13C NMR (CDCl3): d 14.12, 22.69, 26.05, 29.18, 29.35, 29.39, 29.58, 29.60, 29.64, 29.66, 31.92, 68.95, 95.92, 114.39, 120.67, 124.21, 127.82, 133.04, 135.61, 149.01, 152.07, 158.79, 159.70. 4.1.6. 4-(6-Dodecyloxybenzoxazol-2-yl)phenylamine5a (4; n¼12). To a solution of 6-dodecyloxy-2-(4-nitrophenyl)benzoxazole (2.54 g, 0.006 mol) dissolved in absolute C2H5OH (100 mL) was added powdered Pd/C (0.1 g, 0.001 mol) under nitrogen atmosphere. The solution was gently refluxed for 10 min and then N2H4 (0.36 g, 0.007 mol) was added. The mixture was refluxed for 6 h. The solution was filtered while hot. The filtrate was concentrated to give off-white solids. The products isolated as white solids were obtained after recrystallization from C2H5OH, yield 80%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.22–1.46 (m, 18H, –CH2), 1.78–1.81 (m, 2H, –CH2), 3.98 (t, 2H, –OCH2, J¼6.6 Hz), 6.73 (dd, 2H, ArH, J¼1.57, 6.9 Hz), 6.88 (dd, 1H, Ar-H, J¼2.31, 8.7 Hz), 7.04 (d, 1H, Ar-H, J¼2.3 Hz), 7.53 (d, 1H, Ar-H, J¼8.7 Hz), 7.97 (d, 2H, Ar-H, J¼8.6 Hz). 13C NMR
4.1.8. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-fluorobenzylidene)amine (1b; n¼12, X¼F). Light yellow solid, yield 81%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.99 (t, 2H, –OCH2, J¼6.7 Hz), 6.93 (dd, 1H, ArH, J¼2.3 Hz, 8.70 Hz), 7.07 (d, 1H, Ar-H, J¼2.2 Hz), 7.16 (t, 2H, Ar-H, J¼8.6 Hz), 7.29 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.91 (dd, 2H, Ar-H, J¼5.6, 8.6 Hz), 8.20 (d, 2H, Ar-H, J¼8.5 Hz), 8.44 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.36, 29.42, 29.59, 29.61, 29.65, 29.67, 31.93, 68.91, 96.09, 113.35, 115.99, 116.16, 119.81, 121.41, 124.96, 128.30, 131.02, 131.09, 132.31, 132.33, 135.85, 151.67, 154.15, 157.81, 159.58, 161.98, 163.95, 165.97. MS (FAB): calcd for C32H38FN2O2: 500.3, found: 501.4 [M+H]+. Anal. Calcd for C32H37FN2O2: C, 76.77; H, 7.45; N, 5.60. Found: C, 76.48; H, 7.59; N, 5.45. 4.1.9. 4-Chlorobenzylidene-4-(6-dodecyloxybenzoxazol2-yl)phenylamine (1c; n¼12, X¼Cl). Light yellow solid, yield 84%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.8 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, ArH, J¼2.29, 8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.2 Hz), 7.30 (t, 2H, Ar-H, J¼8.5 Hz), 7.45 (d, 2H, Ar-H, J¼8.4 Hz), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.85 (d, 2H, Ar-H, J¼8.4 Hz), 8.21 (d, 2H, Ar-H, J¼8.4 Hz), 8.45 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.36, 29.42, 29.59, 29.62, 29.65, 29.68, 31.93, 68.90, 96.08, 113.37, 114.69, 119.83, 121.43, 125.12, 128.30, 128.92, 129.19, 130.16, 134.43, 135.84, 137.86, 151.68, 153.96, 157.82, 159.60, 161.93. IR (KBr): 2918, 2852, 1612, 1503, 1489,
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1472, 1361, 1321, 1230, 1223, 1148, 1052, 1011, 973, 852, 815 cm1. MS (FAB): calcd for C32H38ClN2O2: 516.3, found: 517.3 [M+H]+. Anal. Calcd for C32H37ClN2O2: C, 74.33; H, 7.21; N, 5.42. Found: C, 74.35; H, 7.40; N, 5.31. 4.1.10. 4-Bromobenzylidene-4-(6-dodecyloxybenzoxazol2-yl)phenylamine (1d; n¼12, X¼Br). Light yellow solid, yield 83%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.84 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, ArH, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.30 (d, 2H, Ar-H, J¼8.5 Hz), 7.59–7.62 (m, 3H, Ar-H), 7.78 (d, 2H, Ar-H, J¼8.4 Hz), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.43 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.36, 29.41, 29.59, 29.61, 29.65, 29.67, 31.93, 68.91, 96.08, 119.83, 121.42, 125.15, 126.41, 128.31, 130.34, 132.16, 134.83, 135.84, 151.68, 153.94, 157.83, 159.73, 161.93. IR (KBr): 2915, 2848, 1625, 1612, 1583, 1566, 1487, 1470, 1397, 1320, 1299, 1219, 1147, 1114, 1068, 1054, 1009, 847, 820 cm1. MS (FAB): calcd for C32H38BrN2O2: 560.2, found: 561.2 [M+H]+. Anal. Calcd for C32H37BrN2O2: C, 68.44; H, 6.64; N, 4.99. Found: C, 68.67; H, 6.63; N, 4.73. 4.1.11. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-methylbenzylidene)amine (1e; n¼12, X¼CH3). Light yellow solid, yield 82%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.7 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –OCH2CH2), 2.41 (s, 3H, Ar-CH3), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.0, 8.65 Hz), 7.07 (d, 1H, Ar-H, J¼1.9 Hz), 7.27–7.30 (m, 4H, Ar-H), 7.60 (d, 2H, Ar-H, J¼8.7 Hz), 7.80 (d, 2H, Ar-H, J¼7.9 Hz), 8.20 (d, 2H, Ar-H, J¼8.4 Hz), 8.44 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.14, 21.70, 22.71, 26.08, 29.26, 29.37, 29.42, 29.60, 29.62, 29.66, 29.68, 31.94, 68.89, 96.07, 113.30, 119.78, 121.46, 124.67, 128.27, 129.05, 129.62, 133.40, 135.86, 142.42, 151.66, 154.62, 157.75, 161.14, 162.10. IR (KBr): 2919, 2848, 1628, 1507, 1489, 1466, 1347, 1282, 1227, 1152, 1137, 1052, 1008, 969, 861, 844, 810, 800 cm1. MS (FAB): calcd for C32H41N2O2: 496.3, found: 497.3 [M+H]+. Anal. Calcd for C32H40N2O2: C, 79.80; H, 8.12; N, 5.64. Found: C, 79.84; H, 8.57; N, 5.30. 4.1.12. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-trifluoromethylbenzylidene)amine (1f; n¼12, X¼CF 3 ). Light yellow solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.94 (dd, 1H, Ar-H, J¼2.20, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.1 Hz), 7.32 (d, 2H, Ar-H, J¼8.4 Hz), 7.60 (d, 1H, ArH, J¼8.7 Hz), 7.73 (d, 2H, Ar-H, J¼8.1 Hz), 8.02 (d, 2H, Ar-H, J¼8.0 Hz), 8.22 (d, 2H, Ar-H, J¼8.5 Hz), 8.52 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.37, 29.42, 29.60, 29.62, 29.66, 29.68, 31.94, 68.89, 96.05, 113.41, 119.87, 121.47, 122.75, 124.91, 125.48, 125.80, 125.83, 128.32, 129.17, 132.97, 133.23, 135.80, 138.95, 151.69, 153.57, 157.87, 159.37, 161.82. IR (KBr): 2918, 2849, 1623, 1610, 1575, 1487, 1470, 1361, 1332, 1299, 1225, 1178, 1169, 1148, 1131, 1106, 1069, 1014, 860, 842, 816 cm1. MS (FAB): calcd for C33H38F3N2O2: 550.3, found: 551.3 [M+H]+. Anal. Calcd
for C33H37F3N2O2: C, 71.78; H, 6.59; N, 4.82. Found: C, 71.98; H, 6.77; N, 5.09. 4.1.13. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-methoxybenzylidene)amine (1g; n¼12, X¼OCH3). Light yellow solid, yield 82%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.77–1.83 (m, 2H, –CH2), 3.86 (s, 3H, Ar-OCH3), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.4, 8.7 Hz), 6.98 (d, 2H, Ar-H, J¼8.8 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.28 (d, 2H, Ar-H, J¼7.0 Hz), 7.59 (d, 1H, Ar-H, J¼8.70 Hz), 7.85 (d, 2H, Ar-H, J¼8.73 Hz), 8.19 (d, 2H, Ar-H, J¼8.53 Hz), 8.40 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.08, 29.26, 29.36, 29.42, 29.60, 29.62, 29.66, 29.68, 31.93, 55.47, 68.90, 96.08, 113.28, 114.30, 119.75, 121.46, 124.48, 128.27, 128.99, 130.82, 135.88, 151.66, 154.74, 157.73, 160.43, 162.15, 162.61. IR (KBr): 2920, 2852, 1607, 1591, 1574, 1511, 1488, 1470, 1320, 1301, 1256, 1223, 1172, 1148, 1052, 1029, 1005, 860, 836, 815 cm1. MS (FAB): calcd for C33H41N2O3: 512.3, found: 513.4 [M+H]+. Anal. Calcd for C33H40N2O3: C, 77.14; H, 7.59; N, 5.20. Found: C, 77.31; H, 7.86; N, 5.46. 4.1.14. [4-(6-Dodecyloxybenzoxazol-2-yl)phenyl]-(4-nitrobenzylidene)amine (1h; n¼12, X¼NO2). Bright yellow solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.86 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2–, J¼6.6 Hz), 6.94 (dd, 1H, ArH, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.35 (d, 2H, Ar-H, J¼8.5 Hz), 7.61 (d, 1H, Ar-H, J¼8.7 Hz), 8.08 (d, 2H, Ar-H, J¼8.8 Hz), 8.24 (d, 2H, Ar-H, J¼8.5 Hz), 8.33 (d, 2H, Ar-H, J¼8.7 Hz), 8.58 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.12, 22.70, 26.07, 29.24, 29.36, 29.41, 29.59, 29.61, 29.65, 29.67, 31.93, 68.92, 96.08, 113.49, 119.93, 121.54, 124.10, 125.95, 128.37, 129.63, 135.79, 141.22, 149.53, 151.72, 153.09, 157.95, 158.24, 161.68. IR (KBr): 2925, 2848, 1714, 1680, 1623, 1611, 1583, 1574, 1551, 1488, 1471, 1362, 1345, 1323, 1284, 1221, 1150, 1113, 1004, 971, 858, 846, 828 cm1. MS (FAB): calcd for C32H38N3O4: 527.3, found: 528.3 [M+H]+. Anal. Calcd for C32H37N3O4: C, 72.84; H, 7.07; N, 7.96. Found: C, 73.01; H, 6.99; N, 7.81. 4.1.15. 4-{[4-(6-Dodecyloxybenzoxazol-2-yl)phenylimino]methyl}benzonitrile (1i; n¼12, X¼CN). Light yellow solid, yield 86%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.80– 1.83 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.3 Hz), 7.29 (d, 2H, Ar-H, J¼9.6 Hz), 7.59–7.62 (m, 3H, Ar-H), 7.77 (d, 2H, Ar-H, J¼8.4 Hz), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.43 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.24, 29.36, 29.41, 29.59, 29.61, 29.65, 29.67, 31.93, 68.92, 96.08, 113.47, 114.82, 118.35, 119.91, 121.50, 125.81, 128.36, 129.30, 132.62, 135.80, 139.63, 151.71, 153.21, 157.93, 158.73, 161.72. IR (KBr): 2921, 2849, 1714, 1681, 1626, 1613, 1495, 1488, 1467, 1361, 1281, 1223, 1152, 1135, 1050, 1010, 971, 861, 847, 799 cm1. MS (FAB): calcd for C33H38N3O2: 507.3, found: 508.3 [M+H]+. Anal. Calcd for C33H37N3O2: C, 78.07; H, 7.35; N, 8.28. Found: C, 78.04; H, 7.23; N, 8.27.
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4.1.16. [4-(6-Dodecyloxybenzoxazol-2-yl)phenyl]-(4-dimethylaminebenzylidene)amine (1k; n¼12, X¼NMe2). Yellow solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.05 (s, 6H, –NMe2), 3.99 (t, 2H, –OCH2, J¼6.6 Hz), 6.72 (d, 2H, Ar-H, J¼8.9 Hz), 6.92 (dd, 1H, Ar-H, J¼2.3, 8.70 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.28 (d, 2H, Ar-H, J¼8.4 Hz), 7.59 (d, 1H, Ar-H, J¼8.7 Hz), 7.77 (d, 2H, Ar-H, J¼8.5 Hz), 8.18 (d, 2H, Ar-H, J¼8.5 Hz), 8.34 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.14, 22.71, 26.08, 29.27, 29.37, 29.43, 29.60, 29.62, 29.66, 29.68, 31.94, 40.16, 68.89, 96.08, 111.56, 113.19, 119.68, 121.54, 123.86, 124.04, 128.24, 130.84, 135.94, 151.64, 152.79, 155.36, 157.64, 160.84, 162.36. IR (KBr): 2920, 2846, 1626, 1609, 1583, 1552, 1527, 1489, 1470, 1434, 1415, 1361, 1283, 1225, 1178, 1164, 1152, 1131, 1045, 1018, 1008, 941, 862, 839, 816 cm1. MS (FAB): calcd for C34H44N3O2: 525.3, found: 526.4 [M+H]+. Anal. Calcd for C34H43N3O2: C, 77.68; H, 8.24; N, 7.99. Found: C, 77.63; H, 8.39; N, 8.31. 4.1.17. 4-{[4-(6-Dodecyloxybenzoxazol-2-yl)phenylimino]methyl}benzoic acid methyl ester (1l; n¼12, X¼COOCH3). Light yellow solid, yield 78%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.46 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.93 (s, 3H, Ar– COOCH3–), 3.99 (t, 2H, –OCH2, J¼6.6 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.32 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.97 (d, 2H, Ar-H, J¼8.3 Hz), 8.13 (d, 2H, Ar-H, J¼8.3 Hz), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.53 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.14, 22.70, 26.07, 29.25, 29.37, 29.42, 29.60, 29.62, 29.66, 29.68, 31.93, 52.39, 68.89, 96.05, 113.40, 119.86, 121.49, 125.39, 128.85, 130.05, 132.68, 135.81, 139.66, 151.68, 153.76, 157.85, 159.92, 161.86, 166.51. IR (KBr): 2954, 2919, 2847, 1717, 1626, 1489, 1466, 1434, 1347, 1285, 1226, 1152, 1136, 1115, 1052, 1010, 968, 957, 854, 841, 800 cm1. MS (FAB): calcd for C34H41N2O4: 540.3, found: 541.3 [M+H]+. Anal. Calcd for C34H40N2O4: C, 75.53; H, 7.46; N, 5.18. Found: C, 75.36; H, 7.35; N, 4.90. 4.1.18. (4-Methoxybenzylidene)-[4-(6-octyloxybenzoxazol-2-yl)phenyl]amine (2a; n¼8, X¼OCH3). Light yellow solid, yield 77%. 1H NMR (CDCl3): d 0.87 (t, 3H, –CH3, J¼6.9 Hz), 1.28–1.47 (m, 10H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.86 (s, 3H, Ar-OCH3), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 6.98 (d, 2H, Ar-H, J¼8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.28 (d, 2H, Ar-H, J¼6.7 Hz), 7.59 (d, 1H, Ar-H, J¼8.7 Hz), 7.85 (d, 2H, Ar-H, J¼8.7 Hz), 8.19 (d, 2H, Ar-H, J¼8.5 Hz), 8.40 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.12, 22.67, 26.08, 29.26, 29.38, 31.83, 55.47, 68.90, 96.08, 113.28, 114.30, 119.75, 121.47, 124.48, 128.27, 128.98, 130.82, 135.88, 151.66, 154.74, 157.73, 160.44, 162.15, 162.62. Anal. Calcd for C29H32N2O3: C, 76.29; H, 7.06; N, 6.14. Found: C, 75.94; H, 7.20; N, 5.91. 4.1.19. [4-(6-Hexadecyloxybenzoxazol-2-yl)phenyl]-(4methoxybenzylidene)amine (2a; n¼16, X¼OCH3). Light yellow solid, yield 82%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.48 (m, 26H, –CH2), 1.79– 1.82 (m, 2H, –OCH2), 3.87 (s, 3H, Ar-OCH3), 3.99 (t, 2H,
9391
–OCH2, J¼6.6 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.71 Hz), 6.98 (d, 2H, Ar-H, J¼8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.3 Hz), 7.29 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, ArH, J¼8.7 Hz), 7.86 (d, 2H, Ar-H, J¼8.7 Hz), 8.19 (d, 2H, Ar-H, J¼8.9 Hz), 8.41 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.08, 29.26, 29.37, 29.42, 29.60, 29.62, 29.68, 29.71, 31.94, 55.47, 68.89, 96.08, 113.28, 114.31, 119.76, 121.46, 124.51, 128.27, 128.92, 130.85, 135.89, 151.66, 157.73, 160.41, 162.14. Anal. Calcd for C37H48N2O3: C, 78.13; H, 8.51; N, 4.93. Found: C, 76.80; H, 8.99; N, 4.73. 4.1.20. 4-{[4-(6-Octyloxybenzoxazol-2-yl)phenylimino]methyl}benzonitrile (2b; n¼8, X¼CN). Light yellow solid, yield 80%. 1H NMR (CDCl3): d 0.88 (t, 3H, –CH3, J¼6.9 Hz), 1.28–1.49 (m, 10H, –CH2), 1.78–1.83 (m, 2H, –OCH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.3 Hz), 7.33 (d, 2H, Ar-H, J¼6.1 Hz), 7.61 (d, 1H, Ar-H, J¼8.7 Hz), 7.77 (d, 2H, Ar-H, J¼8.3 Hz), 8.02 (d, 2H, Ar-H, J¼8.3 Hz), 8.23 (d, 2H, Ar-H, J¼8.8 Hz), 8.53 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.11, 22.67, 26.08, 29.25, 29.37, 31.83, 68.92, 96.07, 113.47, 114.70, 114.83, 118.36, 119.91, 121.50, 125.81, 128.36, 129.30, 129.89, 132.63, 132.91, 135.80, 139.63, 151.72, 153.22, 157.93, 158.73, 161.72. Anal. Calcd for C29H29N3O2: C, 77.13; H, 6.47; N, 9.31. Found: C, 77.01; H, 6.45; N, 9.15. 4.1.21. 4-{[4-(6-Hexadecyloxybenzoxazol-2-yl)phenylimino]methyl}benzonitrile (2b; n¼16, X¼CN). Light yellow solid, yield 87%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.48 (m, 26H, –CH2), 1.79–1.82 (m, 2H, –OCH2), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.2 Hz), 7.32 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, ArH, J¼8.7 Hz), 7.75 (d, 2H, Ar-H, J¼8.2 Hz), 8.00 (d, 2H, Ar-H, J¼8.3 Hz), 8.22 (d, 2H, Ar-H, J¼8.5 Hz), 8.50 (s, 1H, Ar-CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.37, 29.42, 29.59, 29.62, 29.67, 29.71, 31.94, 68.90, 96.06, 113.46, 114.79, 118.34, 119.90, 121.50, 125.79, 128.34, 129.29, 132.60, 135.78, 139.61, 151.70, 153.17, 157.92, 158.69, 161.70. Anal. Calcd for C37H45N3O2: C, 78.83; H, 8.05; N, 7.45. Found: C, 78.83; H, 8.16; N, 7.34. Acknowledgements We thank the National Science Council of Taiwan, ROC and the UST for funding (NSC-94-2113-M-008-011) in generous support of this work. References and notes 1. (a) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304– 1319; (b) Lehn, J. M. Science 1993, 260, 1762–1763; (c) Zerkowski, J. A.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 4298–4304; (d) Geib, S. J.; Vicent, C.; Fan, E.; Hamilton, A. D. Angew. Chem., Int. Ed. Engl. 1993, 32, 119– 121. 2. (a) Vill, V. Landold-Brnstein, New Series; Thiem, J., Ed.; Springer: Berlin, 1992; Vol. 7; (b) Demus, D.; Goodby, G.; Gray, G. W.; Spiess, H. W.; Vill, V. Handbook of Liquid
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The effect of polar substituents on the heterocyclic benzoxazoles Chung-Shu Wang,a I-Wen Wang,a Kung-Lung Chengb and Chung K. Laia,* a
Department of Chemistry and Center for Nano Science Technology, National Central University, Chung-Li 320, Taiwan, ROC b Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu 300, Taiwan, ROC Received 22 May 2006; revised 3 July 2006; accepted 20 July 2006 Available online 10 August 2006
Abstract—The synthesis, characterization, and mesomorphic properties of a new type of heterocyclic compounds 1, 2 derived from benzoxazole are reported. In order to understand the relationship between the structure and the mesomorphic behavior, compounds containing a variety of polar substituents (i.e., X¼H, F, Cl, Br, CH3, CF3, OCH3, NO2, CN, OH, NMe2, COOCH3) on the terminal end were prepared. The phase behavior of these mesogenic compounds was characterized and studied by differential scanning calorimetry (DSC) and polarization optical microscopy. The formation of mesophases was strongly dependent on the electronic and/or the steric factors of the substituents. In general, a mesophase was better induced by introduction of a polar substituent. Compounds (X¼H) formed a crystalline phase, however, other compounds, except for X¼OH, exhibited nematic or smectic A phases. Interestingly all compounds with electron-donating substituents (X¼CH3, OCH3, NMe2) exhibited nematic phases, however, other compounds with electron-withdrawing substituents (X¼F, Cl, Br, CF3, NO2, CN, COOCH3) formed smectic A phases. Compounds (X¼NO2, CN, COOCH3) have higher clearing temperatures than those of other homologues, and the higher Tcl was attributed to an enhanced conjugative interaction. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett sp constants was found. The fluorescent properties of the compounds were examined. All lmax peaks of the absorption and photoluminescence spectra of compounds occurred at ca. 348–381 and 389–478 nm, respectively. Whereas, the quantum yields of some compounds were relatively low. Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction A delicate balance of molecular interactions in a specific state1 is crucially essential to the formation of a predesigned structure. This is particularly important in liquid crystalline materials, and liquid crystals form a state of matter intermediate between the solid and the liquid. This type of molecular interaction1c,d induced to form mesophases can be weak dipole–dipole interaction, dispersion, H-bonding, coordinative force, and others. Forces which are too strong or too weak can lead to the formation of a solid or liquid state. Numerous heterocyclic compounds2 derived from compounds such as 1,3,4-oxadiazole,3 1,2,4-triazole,4 and benzoxazole5 exhibiting interesting mesophases have recently been explored due to their varieties in structures and/or known chemistry. Better mesomorphic behavior6 formed by these heterocyclic structures was attributed to their unsaturation and/or more polarizable nature. On the other hand, a lower symmetry3 and/or non-planar structure caused by nitrogen, oxygen, sulfur, or other atoms incorporated on
* Corresponding author. Tel.: +886 3 4259207; fax: +886 3 4277972; e-mail: [email protected] 0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.07.058
such heterocyclic rings also explained the preferred or better mesomorphic properties. Materials with lower melting temperatures, potential candidates for further applications, are often generated by such compounds. It is well known that the stability7 of the mesophase may be augmented by an increase of the polarity or polarization along the mesogenic core of the molecule. A substantial change of the micro- as well as macro-polarizability in a specific structure or molecule can be easily achieved by the introduction of polar substituent along the preferred molecular direction. The effect of polar substituents in a variety of mesogenic systems8 has been studied and investigated during the past years. Some predictions were successfully made in terms of mesomorphic behavior. The transition temperature of M/I has been correlated with the polarizability anisotropy of bonds to the substituent in 4-(4-substituted phenylazo)phenyl-4-alkoxybenzoates.8d Previous studies7,8c also showed that compounds substituted with polar groups, such as –NO2, –CN, might lead to a higher clearing temperature, which has been attributed to the conjugative interaction increase between the substituent and the ester moiety. On the other hand, the effect of polar substituents on the mesogenic 1,3,4oxadiazoles-based materials was also studied. The mesophase and optical properties of these oxadiazole materials3a were also found to be strongly influenced by the presence of a terminal polar group.
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C12H25O
X
RO
b
C12H25O
c
Y
C12H25O
NO2
NH2
OH
OH
N N
OH
RO O
O
O Cu
O
O
OR
d
a
X
H2n+1CnO
HO
OH
X = Me, OMe, F, Cl, CN, NO2
NO2 C12H25O
Y
OH
OR X
N e
OR
R = C12H25; X = H, F; Y = H, F, CF3, OCH3, NMe2
Benzoxazoles, as an important type of heterocyclic compound, have been studied in a variety of research areas, including non-linear optics (NLO),9 organic light-emitting diodes (OLED),10 and polymeric materials.11 However, benzoxazole derivatives exhibiting mesophases5a,12 have been less often reported. Nematic or/and smectic C mesophases formed by these rod-like compounds were observed. Benzothiazole-derived compounds were also found to have photophysical and fluorescent applications.13 In a previous paper, the substituent effect on the columnar formation in b-diketonate metallomesogens14 was investigated. A rectangular columnar phase (Colr) was observed for substituents with bulkier groups (i.e., X¼Me, Et), however, a hexagonal columnar phase (Colh) was observed for substituents with electron-withdrawing groups (i.e., X¼Cl, Br, I). Interestingly, a satisfactory linear relationship with a correlation coefficient of 0.974 between the clearing temperatures and Hammett sp constants was obtained for the copper complexes.
C12H25O
O NH2
f
C12H25O
O NO2
N
N
4
3
g C12H25O
O N N
X 1a-1l
X = H, F, Cl, Br, CH3, CF3, OCH3, NO2, CN, OH, NMe2, COOCH3
Scheme 1. Reactions and reagents: (a) RBr (1.1 equiv), KHCO3 (1.3 equiv), KI, refluxed in dry CH3COCH3, 48 h, 60 C; (b) HNO3 (1.2 equiv), NaNO2 (0.2 equiv), stirred at 0 C in CH2Cl2, 12 h, 25 C; (c) N2H4 (1.2 equiv), Pd/C (0.1 equiv), refluxed in absolute C2H5OH, 6 h, 85 C; (d) 4-Nitrobenzaldehyde (1.0 equiv), CH3COOH (drops), refluxed in absolute C2H5OH, 12 h, 90 C; (e) Pb(OAc)4 (1.3 equiv), refluxed in CH2Cl2, 6 h, 85 C; (f) N2H4 (1.2 equiv), Pd/C (0.1 equiv), refluxed in absolute ethanol, 6 h, 80%; (g) substituted benzaldehyde (1.0 equiv), CH3COOH (drops), refluxed in absolute C2H5OH, 12 h, 68–90 C.
2.1. Mesomorphic properties As part of our studies of heterocyclic mesogens, in this paper, we describe the synthesis, characterization, and the mesomorphic properties of a series of calamitic benzoxazole derivatives with various substituents on the terminal phenyl ring. Their fluorescent properties were also examined. Nematic or smectic phases were observed depending on the electronic nature of the substituents. However, the observed optical properties were not sensitive to the substituents except for the compound with an NMe2 group. 2. Results and discussion The synthetic procedures5a for the benzoxazole derivatives 1a–1l are summarized in Scheme 1. The majority of benzoxazole-based derivatives were prepared by the condensation of 2-aminophenols with benzaldehydes or benzoic acid and subsequent intramolecular cyclization. The compounds of 5-dodecyloxy-2-nitrophenol, 2-amino-5-dodecyloxyphenol, 5-dodecyloxy-2-[(4-nitrobenzylidene)amino]phenol, 6-dodecyloxy-2-(4-nitrophenyl)benzoxazole 3, and 4-(6dodecyloxybenzoxazol-2-yl)phenylamine 4 were prepared by reported procedures.5a The final compounds, benzylidene-4-(6-dodecyloxybenzoxazol-2-yl)phenylamine were prepared by the condensation reaction of 4-(6-dodecyloxybenzoxazol-2-yl)phenylamine and appropriate aldehydes in refluxing absolute ethanol with a yield ranging from 67–82%. 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis were used to characterize the intermediates and the final products.
The mesomorphic behavior of compounds 1a–1l was characterized and studied by differential scanning calorimetry (DSC) and polarizing optical microscopy. The phase transitions and thermodynamic data were summarized in Table 1. In order to study the substituent effect on the formation of the mesophases, a variety of compounds with a variety of polar groups (i.e., X¼H, F, Cl, Br, CH3, CF3, OCH3, NO2, CN, OH, NMe2, COOCH3) substituted on the opposite end of the benzoxazole ring were studied. Among these polar groups, some are known as strong p-acceptors (i.e., X¼CN, NO2) or p-donors (X¼Cl, Br, OH), and others are known as intermediate (X¼Cl, COOCH3), besides being classified as electron-donating or electron-withdrawing groups. The results indicated that the formation of mesophases in this type of electron-deficient heterocyclic system was strongly dependent on the electronic and/or the steric factors of the substituents. Compound 1a (X¼H) was in fact non-mesogenic. A transition of crystal-to-isotropic at 99.8 C was observed, and the lack of liquid crystallinity of compound 1a can probably be attributed to the insufficient dipole over the entire molecule. When a polar functional group was substituted on the phenyl ring located on the opposite side to the benzoxale core, the mesomorphic properties were improved. All compounds except for 1j (X¼OH) were truly mesogenic, in which introducing a polar group was used to enhance or/and induce the formation of the mesophases. The results also indicated that compounds (1e, 1g, 1k) with an electron-donating substituent (–CH3, OCH3, NMe2) exhibited nematic (N) phases, however, other
C.-S. Wang et al. / Tetrahedron 62 (2006) 9383–9392
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Table 1. Phase transitions and enthalpies of compounds 1a–1l
99.8 (60.1) Cr
1a
79.8 (60.8) 101.7 (59.2)
1b
Cr 82.4 (2.03)
1c
Cr1
1d
Cr1
Cr2
74.9 (56.7) SmA
Cr2
1e
Cr
1f
Cr
80.2 (13.1) 103.9 (55.4)
Cr1
1h
Cr1
1i
Cr1
92.0 (4.61)
Cr2
108.3 (51.6)
137.0 (0.40) 180.9 (6.22) SmA
88.1 (1.96) 102.9 (8.93) 86.3 (6.08)
Cr2
I
I
179.5 (6.11) 169.5 (0.49)
N 169.2 (0.47)
129.3 (7.85) Cr2
I
137.4 (0.38)
76.3 (51.5) 95.6 (2.28)
192.8 (6.36)
N
90.8 (47.6)
1g
I
194.3 (6.42) SmA
86.7 (56.5)
75.3 (50.4)
I
174.0 (4.24) 173.2 (3.28)
97.9 (59.9)
63.3 (14.6)
155.6 (4.25) 154.7 (4.12)
92.1 (45.3) 74.3 (10.3)
50.0 (25.5)
SmA
I
SmA 127.0 (8.03)
224.8 (2.64) 224.2 (2.66)
120.7 (40.1) SmA 102.7 (30.7)
I
219.1 (3.27)
I
I
217.3 (3.21) 130.7 (16.7) Cr
1j
1k
Cr1
123.3 (34.6)
SmX
156.7 (0.29)
Cr1
103.1 (35.6)
Cr2
115.2 (12.7)
SmA
I
161.0 (0.60) 160.5 (0.53)
156.2 (0.23)
111.8 (33.8)
133.0 (51.6) 1l
N
117.2 (16.4)
I
212.5 (5.68) 210.8 (5.54)
I
Cr1, Cr2¼crystalline phases, SmX¼unidentified smectic phase, SmC¼smectic C phase, SmA¼Smectic A phase, N¼nematic phase, I¼isotropic phase.
compounds (1b, 1c, 1d, 1f, 1h, 1i, 1l) with an electronwithdrawing substituent (–F, –Cl, –Br, –CF3, –NO2, –CN, –COOCH3) formed smectic A (SmA) phases. The N and SmA phases were observed and identified by optical texture, as shown in Figure 1. Among them, compounds 1b, 1c , 1d formed SmA phases, however, compound 1d containing a larger and/or more polarized group (X¼Br) enhanced the SmA phase more than compounds with a smaller and/or less polarized group (X¼F, Cl). The clearing temperature of compound 1d was higher than those of compounds 1b and 1c, i.e., 194.3 C>174.0 C>155.6 C. This order in clearing temperatures and the temperature of mesophase was parallel to the increased polarizability in the order of Br>Cl>F. On the other hand, the temperature range of the mesophase was also wider for compound 1d than those of compounds 1a and 1b, i.e., 96.4 C>81.9 C>53.9 C on heating. Interestingly, among these electron-withdrawing substituents, compounds 1h, 1i, 1l (X¼NO2, CN, COOCH3) appeared to have clearing temperatures higher than those of other homologues, which ranged from 212.5 and 224.8 C. These polar groups are also known as good p-acceptors, and have a more planar structure (i.e., sp or sp2 hybrid orbital). The increase in Tcl might be attributed to two factors. Electronically, a conjugative interaction over the entire molecule was better achieved between the polar substituent and the other side of benzoxazole, and also a better packing arrangement
due to a more overall molecular planar structure may be achieved in the solid and liquid crystal states. The temperature range of mesophases were relatively wide with these three compounds, at ca. 79.5 C (1l)–98.4 C (1i). Among these three substituents, the NO2 group is particularly worthy to note. Compound 1h (X¼NO2) has the highest clearing temperature in the whole series, and this is probably due to NO2 group having the largest dipole. The range of mesophase temperature is relatively high; 98.4 C (1i; CN)>96.4 C (1d; Br)>95.5 C (1h; NO2). The effect of lateral and terminal –NO2 as a substituent in achiral calamitic liquid crystals has been reviewed.8e The results indicated that substantial changes in clearing and/or melting temperatures were obviously observed depending on the molecular structures and the substituted position. A polar NO2 group attached to the hydrocarbon moiety changes substantially the electron affinity of the molecules, and the strong mesomeric effect caused by the NO2 group induces a positive charge on the nitrogen atom and increases the electronegativity of the group.8e,15 The transition enthalpies of SmA/I in the whole series of the compounds, measured with 2.64 kJ/mol (1h)–6.42 kJ/mol (1d), varied slightly with the substituents. On the other hand, compounds 1e and 1f, which are relatively equal in molecular size, exhibited a different mesophase; N phase for 1e and SmA phase for 1f, respectively. On the other hand, a higher Tcl in compound 1f (180.9 C) over 1e (137.4 C) by ca. 43.4 C, as well as a wider mesophase range by DT¼56.6 C
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N 150
SmA SmX
100
Cr 50 0
1a 1b 1c 1d 1e
1f
1g 1h
1i
1j 1k
1l
compds
Figure 2. Bar graph showing the phase behavior of compounds 1a–1l.
a transition of crystal-to-isotropic at 130.7 C was observed. The lack of mesomorphic properties might result from the intermolecular H-bonds formed, which can lead to a stronger molecular interaction between the molecules. Figure 2 showed the bar graph of the transition temperatures of compounds 1a–1l. The relationship between the melting temperatures, the clearing temperatures or the temperature ranges of the mesophases with the electronic properties of the substituents was plotted. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett sp constants for the substituents was found in this system (Fig. 3).
(i.e., 90.1 C for 1f vs 33.5 C for 1e) might be attributed to a better or larger dipole on 1f (CF3) over 1e (CH3). The electronic factor of the substituent, but not the steric factor, might have played a more important role in forming or inducing the phase. Heterocyclic benzoxazole is known as an electron-deficient moiety, and polarization or electron distribution by an electron-donating group, such as –CH3, is better achieved by compound 1f than by compound 1e. Compound 1g formed a N phase. The transition enthalpies of N/I, measured with 0.38 kJ/ mol (1e)–0.60 kJ/mol (1k), were insensitive to the substituents. Compound 1j (X¼OH) was non-mesogenic, and only
240 220
COOCH3
200
temp.
Figure 1. Optical textures observed by 1e (X¼CH3) at 119.0 C (top plate), 1f (X¼CF3) at 165.0 C (middle plate), and 1l (X¼COOCH3) at 175.0 C (bottom plate).
In order to understand the effect of terminal carbon chain length on the formation of mesophase, the compounds 2a (X¼OCH3) and 2b (X¼CN) with a carbon chain length of n¼8 and 16 were also prepared and studied. The phase transitions and thermodynamic data are summarized in Table 2. The mesomorphic properties exhibited by three compounds 2a and 1g with different carbon chain length (n¼8, 12, 16) were compared. Compounds 2a (n¼8) and 1g (n¼12) exhibited nematic phases, however, the compound 2a with a longer carbon chain length (n¼16) formed a N phase and a SmA phase. This phase behavior is expected for rod-like molecules. The melting temperature increased slightly with the carbon chain length, i.e., Tmp: 106.1 C (n¼8)< 108.3 C (n¼12)<112.5 C (n¼16), however, the clearing temperatures decreased with the carbon chain length, i.e., Tcl: 186.0 C (n¼8)>169.5 C (n¼12)>158.1 C (n¼16). The temperature range of the mesophase increased with carbon chain length, DT¼82.9 C (n¼8)>61.2 C>45.6 C (n¼16) (Fig. 4). Elongation of the overall molecular length by increasing the carbon length on the other side of terminal chains led to the formation of more ordered SmA phases beside for N phase at higher temperature. This can probably
180 160
2
Br
140
R = 0.6882
CF3
Cl
OCH3 NMe2
NO2
CN
F CH3
120 100 -0.4
-0.2
0
0.2
0.4
0.6
0.8
1
σp Figure 3. The plot of clearing temperature ( C) with Hammett sp constants of substituents in 1a–1l.
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Table 2. Phase transitions and enthalpies of compounds 1g, 1i, 2a, and 2b
106.1 (43.6) 2a; n = 8
1g;
2a;
Cr
12
Cr1
16
Cr
92.0 (4.61)
Cr2
88.3 (67.6)
Cr
106.2 (27.3)
12
2b;
16
Cr1
N
SmA
187.0
a
86.3 (6.08)
I
I
157.0 (0.63) 233.0 (0.45)
N
I
232.2 (0.41) 219.1 (3.27)
120.7 (40.1) Cr2
169.5 (0.49)
158.1 (0.63) N
195.0a
I 185.3 (0.46)
169.2 (0.47)
153.7 (0.42)
102.9 (8.93) 1i;
186.0 (0.47) N
154.8 (0.43) SmA
126.3 (31.9) 8
108.3 (51.6) 76.3 (51.5)
112.5 (63.4)
2b;
78.0 (42.2)
SmA 120.9 (12.2)
Cr
I
217.3 (3.21)
102.7 (30.7) SmA 100.7 (78.8)
213.7 (6.16)
I
211.9 (6.09)
Cr1, Cr2¼crystalline phases, SmA¼Smectic A phase, N¼nematic phase, I¼isotropic phase. a Determined by optical microscope.
230
temp.
180
N SmA
130
spectra of all compounds 1a–1l were very similar in shape because of their structural similarity. The highest absorption peaks of all compounds were found to be insensitive to the substituents. Interestingly, they all occurred at ca. 348– 365 nm except for two compounds 1h and 1k, for which
Cr2 Cr1
80
0.6 1a 1b 1c 1d 1f 1h 1i 1l
30 0.5 2a(n=8) 1g(n=12) 2a(n=16) 2b(n=8) 1i(n=12) 2b(n=16) 0.4
compds
Figure 4. Bar graph showing the phase behavior of compounds 2a, 2b.
absorbance
-20
be attributed to an enhanced dispersive interaction between the terminal alkoxy chains. Similar mesomorphic behavior was also observed in compounds 2b (n¼8, 16) and 1i (n¼12), and compounds 2b with longer carbon chain length (n¼12, 16) favored the formation of the more ordered SmA over N phase.
0.3
0.2
0.1
0.0 300
350
400
450
500
wavelength (nm) H2n+1CnO
O 0.6
N N
1a 1e 1g 1j 1k
0.5 1g; 1h; 1i; 1j; 1k; 1l;
X = OCH3 X = NO2 X = CN X = OH X = NMe2 X = COOCH3
n = 8, 16 2a; X = OCH3 2b; X = CN
0.4 absorbance
1a; 1b; 1c; 1d; 1e; 1f;
n = 12 X=H X=F X = Cl X = Br X = CH3 X = CF3
X
0.3 0.2 0.1
2.2. Optical properties The UV–vis absorption and PL spectra of the compounds 1a–1l in CH2Cl2 solution are presented in Figures 5 and 6. The data of lmax peaks of UV–vis absorption and PL spectra in CH2Cl2 are listed in Table 3. The absorption and emission
0.0 300
350
400
450
500
wavelength (nm)
Figure 5. Normalized absorption spectra of compounds with electron-withdrawing (top) and electron-donating substituents (bottom) in CH2Cl2.
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300 250 200 intensity
quantum yields of some compounds in CH2Cl2 estimated with anthracene as a standard (4f¼0.27 in hexane) were relatively low. For example, the yields were all ranged between 0.05 and 0.07%.
1a 1b 1c 1d 1f 1h 1i 1l
150
3. Conclusions
100 50 0 400
450
500
550
wavelength (nm)
1a 1e 1g 1j 1k
150 125
The effect of polar substituents on a series of heterocyclic benzoxazoles was systematically studied, and the formation of mesophases was observed depending on the electronic and/or the steric factors of the substituents. In general, the mesophase observed was improved by introducing a polar group. Most compounds (except 1j) exhibited nematic or smectic A phases. Compounds with electron-donating substituents favored nematic phases, however, compounds with electron-withdrawing substituents formed smectic A phases. However, no linear correlation between the clearing temperature or the temperature range of mesophases with Hammett sp constants was found.
intensity
100
4. Experimental section
75
4.1. General
50 25 0 400
450
500
550
wavelength (nm)
Figure 6. Normalized PL spectra of compounds with electron-withdrawing (top) and electron-donating substituents (bottom) in CH2Cl2.
the lmax were all shifted to longer wavelength, by ca. 31 and 33 nm, respectively. The electronic properties of the substituents might not play an important role in the UV absorption spectra. The photoluminescence spectra measured in CH2Cl2 of compounds 1a–1l are also shown in Figure 6. A similar trend was also observed in the photoluminescence spectra. The emission peaks occurred at 386–409 nm except for compounds 1k, which the lmax were slightly shifted to longer wavelength, by ca. 80 nm. The dependence of PL spectra is more sensitive to the electronic factor than that of UV-absorption. However, it is worth noting that the
Table 3. A summary of UV and PL peaks for compounds 1a–1l Compounds sp 1a 1b 1c 1d 1f 1h 1i 1l 1e 1j 1g 1k
0 0.15 0.24 0.26 0.53 0.81 0.71 0.44 0.14 0.22 0.25 0.32
UV (nm)
PL (nm)
273, 313 (sh), 348 382, 398 273, 313 (sh), 348 382, 398 277, 313 (sh), 348 387, 397 277, 313, 352 392 267, 313, 356 372, 389 283 (sh), 309, 379 391 (sh), 409, 425 (sh) 273, 309, 365 391 (sh), 409, 425 273, 309, 362 372, 389 280, 313 (sh), 348 382, 398 280 (sh), 313 (sh), 348 372, 386 284 (sh), 313 (sh), 348 382, 398 319 (sh), 381 478
All chemicals and solvents were reagent grades from Lancaster or Aldrich. Solvents were dried by standard techniques. 1H and 13C NMR spectra were measured on a Bruker AM-300. FTIR spectra were performed on Nicolet Magna-IR 550 spectrometer. DSC thermographs were carried out on a Mettler DSC 821 and calibrated with a pure indium sample (mp¼156.60 C, 28.45 J/K), and all phase transitions are determined by a scan rate of 10.0 /min. Optical polarized microscopy was carried out on an Zeiss AxiaPlan equipped with a hot stage system of Mettler FP90/ FP82HT. The UV–vis absorption and fluorescence spectra were obtained using HITACHI F-4500 or JASCO V-530 spectrometer, and all spectra were recorded at room temperature. Elemental analysis for carbon, hydrogen, and nitrogen were conducted on a Heraeus Vario EL-III elemental analyzer at National Taiwan University. 3-Alkoxyphenols, 5-alkoxy-2-nitrophenols, 2-amino-5alkoxyphenols, 5-alkoxy-2-[(4-nitro-benzylidene)amino]phenols, 6-alkoxy-2-(4-nitrophenyl)benzoxazoles, 4-(6-alkoxybenzoxazol-2-yl)phenylamines were prepared by similar procedures.5a 4.1.1. 3-Dodecyloxyphenol5a (n¼12). White solid, yield 60%. 1H NMR (CDCl3): d 0.87 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.43 (m, 18H, –CH2), 1.72–1.77 (m, 2H, –OCH2CH2), 3.90 (t, 2H, –OCH2, J¼6.6 Hz), 6.39–6.40 (m, 2H, Ar-H), 6.46–6.48 (m, 1H, Ar-H), 7.10 (t, 1H, Ar-H, J¼8.5 Hz). 13 C NMR (CDCl3): d 14.14, 22.71, 26.05, 29.23, 29.37, 29.42, 29.60, 29.63, 29.66, 29.68, 31.94, 68.10, 102.08, 107.10, 107.59, 130.10, 156.68, 160.51. 4.1.2. 5-Dodecyloxy-2-nitrophenol5a (n¼12). Yellow solid, yield 25%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.44 (m, 18H, –CH2), 1.75–1.80 (m, 2H, –CH2), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.47–6.49 (m, 2H, Ar-H), 8.00 (d, 1H, Ar-H, J¼10.1 Hz), 11.02 (s, 1H,
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Ar-OH). 13C NMR (CDCl3): d 14.12, 22.69, 25.87, 28.83, 29.27, 29.35, 29.41, 29.52, 29.57, 29.63, 29.64, 29.71, 31.92, 69.15, 101.80, 109.83, 126.91, 127.54, 158.01, 166.74.
(CDCl3): d 14.15, 22.71, 26.08, 29.28, 29.37, 29.43, 29.60, 29.62, 29.66, 29.68, 31.94, 68.88, 96.13, 112.74, 114.70, 117.27, 119.17, 128.93, 135.96, 149.26, 151.35, 157.19, 162.94.
4.1.3. 2-Amino-5-dodecyloxyphenol5a (n¼12). White solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.41 (m, 18H, –CH2), 1.68–1.73 (m, 2H, –CH2), 3.84 (t, 2H, –OCH2, J¼6.6 Hz), 6.32 (dd, 1H, ArH, J¼2.6 Hz, 8.6 Hz), 6.41 (d, 1H, Ar-H, J¼2.7 Hz), 6.75 (d, 1H, Ar-H, J¼8.6 Hz). 13C NMR (CDCl3): d 13.94, 22.59, 26.02, 29.25, 29.34, 29.51, 29.53, 29.56, 29.58, 31.85, 68.67, 102.72, 106.75, 120.85, 125.58, 148.06, 154.94.
4.1.7. Benzylidene-4-(6-dodecyloxybenzoxazol-2-yl)phenylamine (1a; n¼12, X¼H). A mixture of benzaldehyde (0.10 mL, 0.001 mol) and 4-(6-dodecyloxybenzoxazol-2yl)phenylamine (0.4 g, 0.001 mol) was dissolved in 100 mL of absolute ethanol under nitrogen atmosphere. The solution was refluxed for 10 min, and 0.5 mL of acetic acid was slowly added. The solution was refluxed for 12 h. The brown solids were then collected. The products isolated as light yellow solids were obtained after recrystallization from C2H5OH, yield 78%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.8 Hz), 1.25–1.47 (m, 18H, –CH2), 1.80– 1.83 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, Ar-H, J¼2.3 Hz, 8.7 Hz), 7.09 (d, 1H, Ar-H, J¼2.3 Hz), 7.31 (d, 2H, Ar-H, J¼8.4 Hz), 7.47–7.50 (m, 3H, Ar-H), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.91–7.92 (m, 2H, Ar-H), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.49 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.08, 29.26, 29.36, 29.42, 29.60, 29.62, 29.66, 29.68, 31.94, 68.90, 96.08, 113.33, 119.81, 121.44, 124.88, 128.29, 128.87, 129.04, 131.81, 135.86, 135.94, 151.67, 154.42, 157.78, 161.20, 162.04. IR (KBr): 2919, 2848, 1625, 1614, 1505, 1496, 1490, 1467, 1281, 1226, 1150, 1053, 1004, 968, 861, 840, 800 cm1. MS (FAB): calcd for C32H39N2O2: 482.3, found: 483.4 [M+H]+. Anal. Calcd for C32H38N2O2: C, 79.61; H, 7.79; N, 5.42. Found: C, 79.63; H, 7.94; N, 5.80.
4.1.4. 5-Dodecyloxy-2-[(4-nitrobenzylidene)amino]phenol5a (n¼12). Orange solid, yield 90%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.8 Hz), 1.25–1.44 (m, 18H, –CH2), 1.74–1.79 (m, 2H, –CH2), 3.94 (t, 2H, –OCH2, J¼6.5 Hz), 6.47 (dd, 1H, Ar-H, J¼2.4 Hz, 8.9 Hz), 6.55 (d, 1H, Ar-H, J¼2.5 Hz), 7.31 (d, 1H, Ar-H, J¼8.9 Hz), 7.40 (s, 1H, Ar-OH), 7.99 (d, 2H, Ar-H, J¼8.6 Hz), 8.29 (d, 2H, Ar-H, J¼8.6 Hz), 8.66 (s, 1H, Ar–CH]N–Ar). 13 C NMR (CDCl3): d 14.12, 22.70, 26.02, 29.16, 29.36, 29.38, 29.58, 29.60, 29.65, 29.67, 31.93, 68.39, 100.57, 107.84, 116.45, 124.11, 127.46, 128.73, 141.71, 148.90, 149.83, 154.66, 161.56. 4.1.5. 6-Dodecyloxy-2-(4-nitrophenyl)benzoxazole5a (3; n¼12). The mixture of 5-dodecyloxy-2-[(4-nitrobenzylidene)amino]phenol (3.2 g, 0.007 mol) dissolved in 125 mL of CH2Cl2 and Pb(OAc)4 (4.32 g, 0.01 mol) was refluxed for 4 h. The solution was extracted with 100 mL of CH2Cl2/H2O (1/2), and the organic layers were combined and dried over MgSO4. The solution was concentrated and the brown solids were recrystallized from C2H5OH. The light yellow solids were isolated with a yield of 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.49 (m, 18H, –CH2), 1.74–1.84 (m, 2H, –CH2), 4.01 (t, 2H, –OCH2, J¼6.5 Hz), 6.99 (dd, 1H, Ar-H, J¼2.28, 8.8 Hz), 7.09 (d, 1H, Ar-H, J¼2.2 Hz), 7.65 (d, 1H, Ar-H, J¼8.8 Hz), 8.34 (m, 4H, Ar-H). 13C NMR (CDCl3): d 14.12, 22.69, 26.05, 29.18, 29.35, 29.39, 29.58, 29.60, 29.64, 29.66, 31.92, 68.95, 95.92, 114.39, 120.67, 124.21, 127.82, 133.04, 135.61, 149.01, 152.07, 158.79, 159.70. 4.1.6. 4-(6-Dodecyloxybenzoxazol-2-yl)phenylamine5a (4; n¼12). To a solution of 6-dodecyloxy-2-(4-nitrophenyl)benzoxazole (2.54 g, 0.006 mol) dissolved in absolute C2H5OH (100 mL) was added powdered Pd/C (0.1 g, 0.001 mol) under nitrogen atmosphere. The solution was gently refluxed for 10 min and then N2H4 (0.36 g, 0.007 mol) was added. The mixture was refluxed for 6 h. The solution was filtered while hot. The filtrate was concentrated to give off-white solids. The products isolated as white solids were obtained after recrystallization from C2H5OH, yield 80%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.22–1.46 (m, 18H, –CH2), 1.78–1.81 (m, 2H, –CH2), 3.98 (t, 2H, –OCH2, J¼6.6 Hz), 6.73 (dd, 2H, ArH, J¼1.57, 6.9 Hz), 6.88 (dd, 1H, Ar-H, J¼2.31, 8.7 Hz), 7.04 (d, 1H, Ar-H, J¼2.3 Hz), 7.53 (d, 1H, Ar-H, J¼8.7 Hz), 7.97 (d, 2H, Ar-H, J¼8.6 Hz). 13C NMR
4.1.8. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-fluorobenzylidene)amine (1b; n¼12, X¼F). Light yellow solid, yield 81%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.99 (t, 2H, –OCH2, J¼6.7 Hz), 6.93 (dd, 1H, ArH, J¼2.3 Hz, 8.70 Hz), 7.07 (d, 1H, Ar-H, J¼2.2 Hz), 7.16 (t, 2H, Ar-H, J¼8.6 Hz), 7.29 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.91 (dd, 2H, Ar-H, J¼5.6, 8.6 Hz), 8.20 (d, 2H, Ar-H, J¼8.5 Hz), 8.44 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.36, 29.42, 29.59, 29.61, 29.65, 29.67, 31.93, 68.91, 96.09, 113.35, 115.99, 116.16, 119.81, 121.41, 124.96, 128.30, 131.02, 131.09, 132.31, 132.33, 135.85, 151.67, 154.15, 157.81, 159.58, 161.98, 163.95, 165.97. MS (FAB): calcd for C32H38FN2O2: 500.3, found: 501.4 [M+H]+. Anal. Calcd for C32H37FN2O2: C, 76.77; H, 7.45; N, 5.60. Found: C, 76.48; H, 7.59; N, 5.45. 4.1.9. 4-Chlorobenzylidene-4-(6-dodecyloxybenzoxazol2-yl)phenylamine (1c; n¼12, X¼Cl). Light yellow solid, yield 84%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.8 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, ArH, J¼2.29, 8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.2 Hz), 7.30 (t, 2H, Ar-H, J¼8.5 Hz), 7.45 (d, 2H, Ar-H, J¼8.4 Hz), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.85 (d, 2H, Ar-H, J¼8.4 Hz), 8.21 (d, 2H, Ar-H, J¼8.4 Hz), 8.45 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.36, 29.42, 29.59, 29.62, 29.65, 29.68, 31.93, 68.90, 96.08, 113.37, 114.69, 119.83, 121.43, 125.12, 128.30, 128.92, 129.19, 130.16, 134.43, 135.84, 137.86, 151.68, 153.96, 157.82, 159.60, 161.93. IR (KBr): 2918, 2852, 1612, 1503, 1489,
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1472, 1361, 1321, 1230, 1223, 1148, 1052, 1011, 973, 852, 815 cm1. MS (FAB): calcd for C32H38ClN2O2: 516.3, found: 517.3 [M+H]+. Anal. Calcd for C32H37ClN2O2: C, 74.33; H, 7.21; N, 5.42. Found: C, 74.35; H, 7.40; N, 5.31. 4.1.10. 4-Bromobenzylidene-4-(6-dodecyloxybenzoxazol2-yl)phenylamine (1d; n¼12, X¼Br). Light yellow solid, yield 83%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.84 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, ArH, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.30 (d, 2H, Ar-H, J¼8.5 Hz), 7.59–7.62 (m, 3H, Ar-H), 7.78 (d, 2H, Ar-H, J¼8.4 Hz), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.43 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.36, 29.41, 29.59, 29.61, 29.65, 29.67, 31.93, 68.91, 96.08, 119.83, 121.42, 125.15, 126.41, 128.31, 130.34, 132.16, 134.83, 135.84, 151.68, 153.94, 157.83, 159.73, 161.93. IR (KBr): 2915, 2848, 1625, 1612, 1583, 1566, 1487, 1470, 1397, 1320, 1299, 1219, 1147, 1114, 1068, 1054, 1009, 847, 820 cm1. MS (FAB): calcd for C32H38BrN2O2: 560.2, found: 561.2 [M+H]+. Anal. Calcd for C32H37BrN2O2: C, 68.44; H, 6.64; N, 4.99. Found: C, 68.67; H, 6.63; N, 4.73. 4.1.11. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-methylbenzylidene)amine (1e; n¼12, X¼CH3). Light yellow solid, yield 82%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.7 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –OCH2CH2), 2.41 (s, 3H, Ar-CH3), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.0, 8.65 Hz), 7.07 (d, 1H, Ar-H, J¼1.9 Hz), 7.27–7.30 (m, 4H, Ar-H), 7.60 (d, 2H, Ar-H, J¼8.7 Hz), 7.80 (d, 2H, Ar-H, J¼7.9 Hz), 8.20 (d, 2H, Ar-H, J¼8.4 Hz), 8.44 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.14, 21.70, 22.71, 26.08, 29.26, 29.37, 29.42, 29.60, 29.62, 29.66, 29.68, 31.94, 68.89, 96.07, 113.30, 119.78, 121.46, 124.67, 128.27, 129.05, 129.62, 133.40, 135.86, 142.42, 151.66, 154.62, 157.75, 161.14, 162.10. IR (KBr): 2919, 2848, 1628, 1507, 1489, 1466, 1347, 1282, 1227, 1152, 1137, 1052, 1008, 969, 861, 844, 810, 800 cm1. MS (FAB): calcd for C32H41N2O2: 496.3, found: 497.3 [M+H]+. Anal. Calcd for C32H40N2O2: C, 79.80; H, 8.12; N, 5.64. Found: C, 79.84; H, 8.57; N, 5.30. 4.1.12. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-trifluoromethylbenzylidene)amine (1f; n¼12, X¼CF 3 ). Light yellow solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.94 (dd, 1H, Ar-H, J¼2.20, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.1 Hz), 7.32 (d, 2H, Ar-H, J¼8.4 Hz), 7.60 (d, 1H, ArH, J¼8.7 Hz), 7.73 (d, 2H, Ar-H, J¼8.1 Hz), 8.02 (d, 2H, Ar-H, J¼8.0 Hz), 8.22 (d, 2H, Ar-H, J¼8.5 Hz), 8.52 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.37, 29.42, 29.60, 29.62, 29.66, 29.68, 31.94, 68.89, 96.05, 113.41, 119.87, 121.47, 122.75, 124.91, 125.48, 125.80, 125.83, 128.32, 129.17, 132.97, 133.23, 135.80, 138.95, 151.69, 153.57, 157.87, 159.37, 161.82. IR (KBr): 2918, 2849, 1623, 1610, 1575, 1487, 1470, 1361, 1332, 1299, 1225, 1178, 1169, 1148, 1131, 1106, 1069, 1014, 860, 842, 816 cm1. MS (FAB): calcd for C33H38F3N2O2: 550.3, found: 551.3 [M+H]+. Anal. Calcd
for C33H37F3N2O2: C, 71.78; H, 6.59; N, 4.82. Found: C, 71.98; H, 6.77; N, 5.09. 4.1.13. 4-(6-Dodecyloxybenzoxazol-2-yl)phenyl-(4-methoxybenzylidene)amine (1g; n¼12, X¼OCH3). Light yellow solid, yield 82%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.77–1.83 (m, 2H, –CH2), 3.86 (s, 3H, Ar-OCH3), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.4, 8.7 Hz), 6.98 (d, 2H, Ar-H, J¼8.8 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.28 (d, 2H, Ar-H, J¼7.0 Hz), 7.59 (d, 1H, Ar-H, J¼8.70 Hz), 7.85 (d, 2H, Ar-H, J¼8.73 Hz), 8.19 (d, 2H, Ar-H, J¼8.53 Hz), 8.40 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.08, 29.26, 29.36, 29.42, 29.60, 29.62, 29.66, 29.68, 31.93, 55.47, 68.90, 96.08, 113.28, 114.30, 119.75, 121.46, 124.48, 128.27, 128.99, 130.82, 135.88, 151.66, 154.74, 157.73, 160.43, 162.15, 162.61. IR (KBr): 2920, 2852, 1607, 1591, 1574, 1511, 1488, 1470, 1320, 1301, 1256, 1223, 1172, 1148, 1052, 1029, 1005, 860, 836, 815 cm1. MS (FAB): calcd for C33H41N2O3: 512.3, found: 513.4 [M+H]+. Anal. Calcd for C33H40N2O3: C, 77.14; H, 7.59; N, 5.20. Found: C, 77.31; H, 7.86; N, 5.46. 4.1.14. [4-(6-Dodecyloxybenzoxazol-2-yl)phenyl]-(4-nitrobenzylidene)amine (1h; n¼12, X¼NO2). Bright yellow solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.86 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2–, J¼6.6 Hz), 6.94 (dd, 1H, ArH, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.35 (d, 2H, Ar-H, J¼8.5 Hz), 7.61 (d, 1H, Ar-H, J¼8.7 Hz), 8.08 (d, 2H, Ar-H, J¼8.8 Hz), 8.24 (d, 2H, Ar-H, J¼8.5 Hz), 8.33 (d, 2H, Ar-H, J¼8.7 Hz), 8.58 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.12, 22.70, 26.07, 29.24, 29.36, 29.41, 29.59, 29.61, 29.65, 29.67, 31.93, 68.92, 96.08, 113.49, 119.93, 121.54, 124.10, 125.95, 128.37, 129.63, 135.79, 141.22, 149.53, 151.72, 153.09, 157.95, 158.24, 161.68. IR (KBr): 2925, 2848, 1714, 1680, 1623, 1611, 1583, 1574, 1551, 1488, 1471, 1362, 1345, 1323, 1284, 1221, 1150, 1113, 1004, 971, 858, 846, 828 cm1. MS (FAB): calcd for C32H38N3O4: 527.3, found: 528.3 [M+H]+. Anal. Calcd for C32H37N3O4: C, 72.84; H, 7.07; N, 7.96. Found: C, 73.01; H, 6.99; N, 7.81. 4.1.15. 4-{[4-(6-Dodecyloxybenzoxazol-2-yl)phenylimino]methyl}benzonitrile (1i; n¼12, X¼CN). Light yellow solid, yield 86%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.80– 1.83 (m, 2H, –CH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.3 Hz), 7.29 (d, 2H, Ar-H, J¼9.6 Hz), 7.59–7.62 (m, 3H, Ar-H), 7.77 (d, 2H, Ar-H, J¼8.4 Hz), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.43 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.24, 29.36, 29.41, 29.59, 29.61, 29.65, 29.67, 31.93, 68.92, 96.08, 113.47, 114.82, 118.35, 119.91, 121.50, 125.81, 128.36, 129.30, 132.62, 135.80, 139.63, 151.71, 153.21, 157.93, 158.73, 161.72. IR (KBr): 2921, 2849, 1714, 1681, 1626, 1613, 1495, 1488, 1467, 1361, 1281, 1223, 1152, 1135, 1050, 1010, 971, 861, 847, 799 cm1. MS (FAB): calcd for C33H38N3O2: 507.3, found: 508.3 [M+H]+. Anal. Calcd for C33H37N3O2: C, 78.07; H, 7.35; N, 8.28. Found: C, 78.04; H, 7.23; N, 8.27.
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4.1.16. [4-(6-Dodecyloxybenzoxazol-2-yl)phenyl]-(4-dimethylaminebenzylidene)amine (1k; n¼12, X¼NMe2). Yellow solid, yield 85%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.48 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.05 (s, 6H, –NMe2), 3.99 (t, 2H, –OCH2, J¼6.6 Hz), 6.72 (d, 2H, Ar-H, J¼8.9 Hz), 6.92 (dd, 1H, Ar-H, J¼2.3, 8.70 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.28 (d, 2H, Ar-H, J¼8.4 Hz), 7.59 (d, 1H, Ar-H, J¼8.7 Hz), 7.77 (d, 2H, Ar-H, J¼8.5 Hz), 8.18 (d, 2H, Ar-H, J¼8.5 Hz), 8.34 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.14, 22.71, 26.08, 29.27, 29.37, 29.43, 29.60, 29.62, 29.66, 29.68, 31.94, 40.16, 68.89, 96.08, 111.56, 113.19, 119.68, 121.54, 123.86, 124.04, 128.24, 130.84, 135.94, 151.64, 152.79, 155.36, 157.64, 160.84, 162.36. IR (KBr): 2920, 2846, 1626, 1609, 1583, 1552, 1527, 1489, 1470, 1434, 1415, 1361, 1283, 1225, 1178, 1164, 1152, 1131, 1045, 1018, 1008, 941, 862, 839, 816 cm1. MS (FAB): calcd for C34H44N3O2: 525.3, found: 526.4 [M+H]+. Anal. Calcd for C34H43N3O2: C, 77.68; H, 8.24; N, 7.99. Found: C, 77.63; H, 8.39; N, 8.31. 4.1.17. 4-{[4-(6-Dodecyloxybenzoxazol-2-yl)phenylimino]methyl}benzoic acid methyl ester (1l; n¼12, X¼COOCH3). Light yellow solid, yield 78%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.25–1.46 (m, 18H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.93 (s, 3H, Ar– COOCH3–), 3.99 (t, 2H, –OCH2, J¼6.6 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.32 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, Ar-H, J¼8.7 Hz), 7.97 (d, 2H, Ar-H, J¼8.3 Hz), 8.13 (d, 2H, Ar-H, J¼8.3 Hz), 8.21 (d, 2H, Ar-H, J¼8.5 Hz), 8.53 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.14, 22.70, 26.07, 29.25, 29.37, 29.42, 29.60, 29.62, 29.66, 29.68, 31.93, 52.39, 68.89, 96.05, 113.40, 119.86, 121.49, 125.39, 128.85, 130.05, 132.68, 135.81, 139.66, 151.68, 153.76, 157.85, 159.92, 161.86, 166.51. IR (KBr): 2954, 2919, 2847, 1717, 1626, 1489, 1466, 1434, 1347, 1285, 1226, 1152, 1136, 1115, 1052, 1010, 968, 957, 854, 841, 800 cm1. MS (FAB): calcd for C34H41N2O4: 540.3, found: 541.3 [M+H]+. Anal. Calcd for C34H40N2O4: C, 75.53; H, 7.46; N, 5.18. Found: C, 75.36; H, 7.35; N, 4.90. 4.1.18. (4-Methoxybenzylidene)-[4-(6-octyloxybenzoxazol-2-yl)phenyl]amine (2a; n¼8, X¼OCH3). Light yellow solid, yield 77%. 1H NMR (CDCl3): d 0.87 (t, 3H, –CH3, J¼6.9 Hz), 1.28–1.47 (m, 10H, –CH2), 1.79–1.82 (m, 2H, –CH2), 3.86 (s, 3H, Ar-OCH3), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 6.98 (d, 2H, Ar-H, J¼8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.3 Hz), 7.28 (d, 2H, Ar-H, J¼6.7 Hz), 7.59 (d, 1H, Ar-H, J¼8.7 Hz), 7.85 (d, 2H, Ar-H, J¼8.7 Hz), 8.19 (d, 2H, Ar-H, J¼8.5 Hz), 8.40 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.12, 22.67, 26.08, 29.26, 29.38, 31.83, 55.47, 68.90, 96.08, 113.28, 114.30, 119.75, 121.47, 124.48, 128.27, 128.98, 130.82, 135.88, 151.66, 154.74, 157.73, 160.44, 162.15, 162.62. Anal. Calcd for C29H32N2O3: C, 76.29; H, 7.06; N, 6.14. Found: C, 75.94; H, 7.20; N, 5.91. 4.1.19. [4-(6-Hexadecyloxybenzoxazol-2-yl)phenyl]-(4methoxybenzylidene)amine (2a; n¼16, X¼OCH3). Light yellow solid, yield 82%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.48 (m, 26H, –CH2), 1.79– 1.82 (m, 2H, –OCH2), 3.87 (s, 3H, Ar-OCH3), 3.99 (t, 2H,
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–OCH2, J¼6.6 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.71 Hz), 6.98 (d, 2H, Ar-H, J¼8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.3 Hz), 7.29 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, ArH, J¼8.7 Hz), 7.86 (d, 2H, Ar-H, J¼8.7 Hz), 8.19 (d, 2H, Ar-H, J¼8.9 Hz), 8.41 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.08, 29.26, 29.37, 29.42, 29.60, 29.62, 29.68, 29.71, 31.94, 55.47, 68.89, 96.08, 113.28, 114.31, 119.76, 121.46, 124.51, 128.27, 128.92, 130.85, 135.89, 151.66, 157.73, 160.41, 162.14. Anal. Calcd for C37H48N2O3: C, 78.13; H, 8.51; N, 4.93. Found: C, 76.80; H, 8.99; N, 4.73. 4.1.20. 4-{[4-(6-Octyloxybenzoxazol-2-yl)phenylimino]methyl}benzonitrile (2b; n¼8, X¼CN). Light yellow solid, yield 80%. 1H NMR (CDCl3): d 0.88 (t, 3H, –CH3, J¼6.9 Hz), 1.28–1.49 (m, 10H, –CH2), 1.78–1.83 (m, 2H, –OCH2), 4.00 (t, 2H, –OCH2, J¼6.6 Hz), 6.94 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.08 (d, 1H, Ar-H, J¼2.3 Hz), 7.33 (d, 2H, Ar-H, J¼6.1 Hz), 7.61 (d, 1H, Ar-H, J¼8.7 Hz), 7.77 (d, 2H, Ar-H, J¼8.3 Hz), 8.02 (d, 2H, Ar-H, J¼8.3 Hz), 8.23 (d, 2H, Ar-H, J¼8.8 Hz), 8.53 (s, 1H, –CHN). 13C NMR (CDCl3): d 14.11, 22.67, 26.08, 29.25, 29.37, 31.83, 68.92, 96.07, 113.47, 114.70, 114.83, 118.36, 119.91, 121.50, 125.81, 128.36, 129.30, 129.89, 132.63, 132.91, 135.80, 139.63, 151.72, 153.22, 157.93, 158.73, 161.72. Anal. Calcd for C29H29N3O2: C, 77.13; H, 6.47; N, 9.31. Found: C, 77.01; H, 6.45; N, 9.15. 4.1.21. 4-{[4-(6-Hexadecyloxybenzoxazol-2-yl)phenylimino]methyl}benzonitrile (2b; n¼16, X¼CN). Light yellow solid, yield 87%. 1H NMR (CDCl3): d 0.86 (t, 3H, –CH3, J¼6.9 Hz), 1.24–1.48 (m, 26H, –CH2), 1.79–1.82 (m, 2H, –OCH2), 3.99 (t, 2H, –OCH2, J¼6.5 Hz), 6.93 (dd, 1H, Ar-H, J¼2.3, 8.7 Hz), 7.07 (d, 1H, Ar-H, J¼2.2 Hz), 7.32 (d, 2H, Ar-H, J¼8.5 Hz), 7.60 (d, 1H, ArH, J¼8.7 Hz), 7.75 (d, 2H, Ar-H, J¼8.2 Hz), 8.00 (d, 2H, Ar-H, J¼8.3 Hz), 8.22 (d, 2H, Ar-H, J¼8.5 Hz), 8.50 (s, 1H, Ar-CHN). 13C NMR (CDCl3): d 14.13, 22.70, 26.07, 29.25, 29.37, 29.42, 29.59, 29.62, 29.67, 29.71, 31.94, 68.90, 96.06, 113.46, 114.79, 118.34, 119.90, 121.50, 125.79, 128.34, 129.29, 132.60, 135.78, 139.61, 151.70, 153.17, 157.92, 158.69, 161.70. Anal. Calcd for C37H45N3O2: C, 78.83; H, 8.05; N, 7.45. Found: C, 78.83; H, 8.16; N, 7.34. Acknowledgements We thank the National Science Council of Taiwan, ROC and the UST for funding (NSC-94-2113-M-008-011) in generous support of this work. References and notes 1. (a) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304– 1319; (b) Lehn, J. M. Science 1993, 260, 1762–1763; (c) Zerkowski, J. A.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 4298–4304; (d) Geib, S. J.; Vicent, C.; Fan, E.; Hamilton, A. D. Angew. Chem., Int. Ed. Engl. 1993, 32, 119– 121. 2. (a) Vill, V. Landold-Brnstein, New Series; Thiem, J., Ed.; Springer: Berlin, 1992; Vol. 7; (b) Demus, D.; Goodby, G.; Gray, G. W.; Spiess, H. W.; Vill, V. Handbook of Liquid
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